moving coil vs. moving magnet question

As someone who is just getting back into the whole "audiophile" thing, I'm wondering what kind of cartridge to get. My Rega P3 with Grace 727 tonearm presently has an old Supex 900 low output moving coil cartridge that I'd like to replace. I use a PS audio step-up for it, but wonder whether I'd be disappointed if I went to a MM cartridge, e.g. a Shure V15v. What's the sonic difference between MC and MM?
Phono cartridges are transducers that convert mechanical movement into electrical energy. The other transducer in your audio system is your speakers, which operate in the reverse fashion.

In a nutshell, the main difference between moving coil (MC) and moving magnet (MM) cartridges lies in the construction of the transducer assembly. The three main elements in the transducer assembly are the stylus/cantilever, the coil, and the magnet. The electrical engineering types can give you a more technically sophisticated answer, but here is the simple version.

MC cartridges have the coil wound around the end of the cantilever, and this coil moves within the magnet's field to generate a voltage signal. Because the coil is wrapped around the end of the cantilever, there isn't a lot of room for numerous, heavy wraps. There is a plus and a minus with this approach: the plus is that the low mass of the coil allows it to move very quickly as the stylus traces the LP's grooves. The minus is that the small number of coil windings results in a low voltage output, which means that the signal must often go through a step-up transformer or "pre-preamp" before going to the main preamp. The theoretical advantage of the MC cartridge is greater transient speed, and sometimes more extended high frequencies.

Moving magnet (MM) cartridges essentially have the coil and magnet assembly reversed from that of the MC. The magnet structure is located at the end of the cantilever, and it moves within a coil assembly. Since the coil can be larger (i.e., have more windings), the signal generated by the movement of the cantilever is higher. MM cartridges, therefore, typically generate a much larger voltage than MC's, which negates the need for a step-up device before the main preamp. The minus, if there is one, is slower transient speed, and (maybe) less extended high frequency response.

Having said all of that, the relative advantages and disadvantages of each cartridge type tend to be less noticeable with lower-priced models. The Shure V15VxMR you mention is a fine cartridge that performs very well at its price range. Like many audiophiles, I have owned a number of Shure V15's over the years, and recommend them without hesitation to someone who wants a lot of performance for the money.

There are other options, however, that you may want to consider. The Grado line of cartridges also offers a lot, particularly in its lower-priced models (in the $100-300 range). The Grados are usually classified as a "moving iron" cartridge, but they operate like a MM, and usually have an output signal comparable to the MM cartridge.

You will often get a lot of spirited debate between the MC vs. MM cartridges advocates, but except for the really pricey models ($1000 and up), the sonic differences are fairly subtle. At the lower priced range you seem to be considering, the preamp will be as important to the eventual audio quality as the cartridge itself.
Thank you so much for that detailed response - I really appreciate the lesson. Apparently there are some moving coil cartridges, such as the Sumiko Blue Point Special that a friend recommended to me, that do not require the pre-preamp. Does that mean they have more coil windings around the cantilever, and thus give up some of the advantages offered by the lower output ones?
sd is right on about the performance aspects and/or quality, I enjoyed that thourogh responce. I feel that it should be added that both types have different requirements to get the most out of them, much for the same reasons stated above. phono stages are equally as important as the cartridge, and how the two work together. mm's don't usually require a lot of gain in the phono stage, but they are sensitive to capacitance, of both the cable going into the phono and the phono stage itself. You should be sure to check that to be sure it is within the range of the particular cartridge if you go with a mm. nc's are not affected by capacitance, but the amount of gain must be sufficiant and the loading, the ohms. Low output mc's usually perform better than high output as explained by sd, but then you must have enough gain in the phono stage to reap the benifits. many mc's are designed to run better at a higher loading the mm's, it is typical to find phono stages with both a mm input and a mc input, with a loading of 47k ohms for mm and 100 ohms for mc.
the basis for my responce is that you should check your phono stage to see what it will work well with. the ps audio step up is a good one, and you might actually be geared up really well for a low output mc, especailly since that is what your system had before. there has been a lot of buzz on this site about the denon dl-103, which is a reasonably priced low output mc. the sumiko bps is a good candidate for a lot of set ups because it is a mc that was actually designed to be within the requirements af a mm phono input. I am not familiar with the grace 999, but one grace arm i had set up before was a real light, low mass arm. the bps does not like low mass arms, but the shure you mention does. i'm glad i remembered this important aspect in choosing a cartridge- different cartridges all have requirements as to the mass of the arm based on their complience. most mc's made now are designed for high mass arms while mm's are usually more complient, requiring a lower mass arm.
a good a'goner to seek out is twl, he is very knowlegdeable on the specs of arms, cartridges, and analog in general. many have made post of satisfaction on this site after following his advise.
The Grace is a good medium mass tonearm, the PS audio is probably a good step up still. I'd try a Dynavector, a Shelter, or an Ortofon. What's your price range?
I didn't want to go much beyond the $300 mark, as I'm buying some other new equipment as well. I figured some of my old equipment has been around here for quite awhile, and a change is always fun.
The info in this thread is great, especially that from sdcampbell. I recently posted a question with regard to the MM stage in the McIntosh MA6900. (probably on the wrong forum) I have recently purchased the mac and was wondering whether it would better to buy a new mm cartrige or use some kind of step up device so that I can use my existing Audio Technica OC-9. I am currently working abroad and do not have the turntable with me at the moment (Aplhason Sonata with HR100 arm) I am reliably informed that the phono stage in the mac is pretty good quality, would using a step up device with an MC impede the performance of the macs MM phono stage ? If you guys think it would better to use an MM what do you advise ? One guy has said that the clearaudio vituoso is a good bet also that the Ortofon OM-30 is a good cartridge.
For those who want to pursue the topic of MC vs. MM cartridges a bit further, I suggest you check the Audiogon archives. There have been a number of threads on this topic over the past 2 years, and the various threads should answer any questions you may have.