I don't have as much cartridge experience as some of the posters here, but I have tried the Ortofon OM10E, Shure M97xE, Denon DL-160, and my preference for the past 6 years, the AT150MLX. I find the AT150MLX to be linear and extended, and able to extract a surprising amount of music out of the groove, especially at its price. Any cartridge I know of that others prefer is several hundred dollars more. For example, the Shelter 501 II costs FOUR times as much.
The AT150MLX is built very much as a high end component with top line parts--solid aluminum body with ceramic damping, PCOCC copper winding throughout, low mass tiny dual magnets for quicker response and better separation, metal shielding between the channels, gold-damped boron cantilever, and the MicroLine stylus.
At $325, the AT150MLX is not only an initial-purchase bargain, it is also economical to own. According to my local dealer, the ML stylus shape is unusually durable. Whereas 1,000 hours is the typical life of a stylus, the MicroLine's life is closer to 4,000 hours. That proved to be the case with mine. My AT150MLX's sound didn't dull for five years. I bought a replacement stylus and it still plays like new. Currently the replacement stylus is $179 at LPGear.
To get all that performance at that price, the AT150MLX does have limitations, mostly in loading and setup. It needs a 47Kohm load and a total capacitive load of 100-200 pF, The SL1200's interconnect cable is about 100 pF, so the phono stage should have between 0 and 100 pF input capacitance. Given that it is a high resolution transducer, you will also find that it will be sensitive to tracking force and possibly anti-skate settings. I wouldn't sweat the tracking force too much. Start it at close to 2g to break it in. After it's broken in the 2g will probably sound a bit bass-heavy and you'll want to dial it back to 1.25 to 1.5g where it'll probably stay until you replace it.
I definitely recommend upgrading the headshell. Both the Sumiko and LPGear ZuPreme
headshells come with upgraded cartridge leads. Having both, I like the Zupreme better because the finger lift is mounted to the headshell and makes cartridge mounting easier than on the Sumiko where the finger lift has to be mounted along with the cartrige itself. Its also quite a bit less money at $54.95 vs. $90 for the Sumiko.
As for tonearm rewiring, this new KAB tonearm wire
looks intriguing for both electrical and mechanical performance and is economical to boot.