I recently picked up a 22/45. Out of the box the trigger is HORRIBLE, and magazines won't drop free. With the crapola trigger pull it still shoots good groups. I bought the Volquartsen competition sear and hammer bushing. $30 from Midwayusa...I installed the components yesterday in 20 minutes following a "diy youtube" video made by somebody-"rook." I'll have to check my history to find the video again. My father in law was watching...he's disassembled and reassembled one with only a manual and said that it took hours. He said he couldn't believe how easy it seemed with someone demonstrating. It was my first time ever field stripping a Ruger Mark 22/45, let alone removing internal components, and I would rate it at an "easy" level, now having been led through it.
The 22/45 definitely wins as far as aftermarket parts support and avenues of upgrade. Mags are common and easy to buy locally...I sourced 5 of them prior to my CCW class locally, and this was last month! I also like the fact that the 22/45's that are factory threaded correctly for a suppressor and don't require some gay adapter like the P22, 1911-22's, conversions, and I assume the M&P22. I owned a P22 briefly and couldn't have been happier to dump it, "Jam-0-Matic" was an understatement. I don't know anything about the M&P22 except that if you own an M&P already, I guess it makes sense from a muscle memory standpoint.
I'd go with the 22/45. Removable grips are a must. Avoid the earlier generation where they were molded into the frame. Whoever has accuracy issues with their's needs to send it back to Ruger for a fix. Of all recommendations I received from Bullseye competitors at our club, most recommended the gun to me for competition, and none had a negative comment about them. The usual comment was that you could spend more (sw41, Marvel unit 1, Hammerli) but why would you want to when you can tune a Ruger to shoot with any of the other pistols at a fraction of the cost.
I like mine...but I also like accurate, reliable, easy-to-upgrade firearms with lots of aftermarket support. YMMV. I'm in no way affiliated with Ruger, just a happy owner.