I'm running MP1. With my current build I can blaze through Act 3 MP1.
MP1 vs MP2: Moster health
75% more, damage 16% more. For this you get 10% more XP, 25% more MF and 25% more gold find, and 7% to the bonus item.
Considering the amount of MF I run the 25% bonus is no game changer. 10% more XP when you on 130% odd with 5 stacks is really negligible. Gold find, cute. The bonus item, only affects whites, not so important if you mainly elite hunting. I will admit, that maybe this bonus item could be a big reason to rather do higher MP's and killing all trash, but I'll wait for some more data on that.
The increase damage I could probably live with. But the 75% extra health will slow be down big time and give me problems. Right now a 4 second serenity pretty much ensures I can down most mobs without running the risk of dying. More monster life would totally throw this out of whack and I'd probably be forced to kite every now and again for the nasty mobs.
Well, there you have my reasoning why I'm not only sticking to MP1 for now, but actually geared towards it. I've run higher MPs before, but this for me is just way WAY more efficient.
For me the con's of moving up to a higher MP outweigh the pro's. Remember, apart from the MF bonus and the bonus item drop you stand NO greater chance of picking legendaries and what not moving to a higher MP. To carry on from this line of thinking, if you can farm MP1 easily, wearing tons of (150+) MF gear, good chance you going to see more legendaries drop then if you had to run 0 MF and MP3. IMO.
Gearing an MP1 high MF killing machine might also be easier/cheaper than gearing up a 0 MF MP4 toon.
Man, I'm bored...I could go on and on. hehe