SUT or phono stage?

I have a new Rega P8 turntable on order complete with the lomc Alpheta 2 cartridge. Right now my main turntable is a restored Thorens TD 124 with an SME arm and an AT Art 9 cartridge. My Thorens is running through my McIntosh MC 47 preamp into the MC section and it sounds really nice. I’m going to have two tables hooked up to my system shortly and would like some advice whether I should get a separate phono pre or use an sut into the MM section of my McIntosh preamp. The phono section of my preamp has really good reviews and I can state the the MC section sounds good. I don’t have any experience with a SUT so it would be nice to have some thoughts from those who do. I do have a Vista Audio preamp that’s not bad also.
I suggest that you find (or make) a friend who has an SUT that will match your cartridge and try it.  I'm an SUT guy - I own several for my favorite cartridges and I prefer the sound of MC and an SUT to a high output phono stage or head amp.  But YMMV.  If possible, try before you buy.

Maybe a local dealer will allow you to borrow an SUT (assuming that if you like it you will purchase it from said dealer).  Many online shops (Needle Doctor, Sweetwater, Audio Advisor) will accept returns with little or no restock fee so you don't have to feel guilty about testing.

I thought that the two cartridges have comparable outputs, so they can be switched without any need for more gain when using one instead of the other.  Why would you need an additional SUT or amplification stage?

I really don’t want to unplug one table and plug in the other one. Also the Art 9 is .5mv output and the Rega is .35mv. Even though I live in a city of around a million people there are no dealers for higher end audio so no trying a sut. I’m tempted to buy one to see what they are all about.
Good news is that the specs on your two carts are similar enough (0.35mV/10ohms vs 0.5mV/12ohms) that the same SUT should work for both and therefore you can get one and try whether you like the sound of each cart via the MC or MM inputs best

Like @br3098 I'm a convert to SUTs and like the ones from Music First Audio -- their basic models are very nice and price competitive but there are lots of other sources as well (Bob's for example)
I take it you have two phono  inputs into the McIntosh, one for MM and another for MC so that you can easily switch between the two using a selector. The McIntosh would be providing part of the gain and the equalization for both inputs, so the MM input can be used for a MC if you can supply additional voltage gain without equalization.  I don't know about active gain for this purpose.  I am familiar with using a step up transformer.  I like the sound of decent SUT's.  But, one of the problems with another gain stage or SUT is that you will be adding another box and additional cables and this can somewhat degrade sound and/or introduce noise.  I tried to use an external SUT in my system in lieu of the SUT built into my phono stage and excess hum became a problem.

The other approach would be a MC phonostage (either with enough active gain or a built in SUT) which would also have phono equalization; this would feed an auxiliary input.

I don't know if it is viable to buy a switching box so that you could switch between the two phono rigs prior to the signal feeding the MC input of the McIntosh.  That might work if such a small signal can be fed through the selector switch and additional cabling without substantially degrading the signal or introducing noise. 
Hey Larry. Thanks for your feedback. The Mac preamp does have a MC phono input and a MM one also. I could also use a phono preamp into a line stage. I have a pretty decent inexpensive phono stage, the Vista Audio, which I can use to start with. I’ve been interested in a sut for a while because of what I’ve read online. I would be not inclined to add a switching box into the signal path because it might degrade it. Might keep on the lookout for a nice used sut to try. If I don’t like it I could move it along.
I generally like most of the SUT I've heard.  Stereophile's Art Dudley once said that he never met a SUT he didn't like, even when the particular model did not quite match the output of the cartridge or the source impedance of the cartridge.  I sort of agree.  Proper matching is not that ultra critical.  

If you get a SUT, you can try it out on both cartridges by using the MM input on the Mac.  No matter what, I think you will have some fun if you get a SUT. 

But, a SUT not built into a phono stage has to be positioned properly to avoid picking up noise.  This might mean experimenting with location and orientation of the SUT.   
I run a Miyajami zero low mc mono into a Bob’s device SUT and then into MM phono stage in a MA6900.  Sounds wonderful. Actually I only use half an SUT.   No power cable with a SUT.
My experience is different. With a good phono preamp like the Aesthetix Io, I prefer a direct connection, no SUT.  Although my testing has been limited. It has involved carts in the .25 - .60 mV output range. With a tube phono preamp there will be some noise, and a SUT may be useful for a low output cart, but so far, I have been better served without it (running a .55 ouptut cart, the tube noise is manageable). The SUT, in my case, gave a more beefy and gratifying performance at first sight, but I soon noticed that the phono preamp on its own had more clarity.