One thing none of us asked: do you want to compete with an emphasis on the competition aspect? If so, for IDPA I would go with an M&P Pro9L with a fibre optic front sight and slightly wide rear sight notch.
Or, do you want to see how good you can get with your intended carry gun, without concern for winning competitions? in which case you should go for whatever configuration you prefer, although I would recommend night sights - I have Heinie Straight-8 sights and Apex sears in my M&P45s in full and compact.
As Therealkoop noted, .45 is not optimal for IDPA if winning a match is the goal. 9mm dominates in autos. I tend to run .45 anyway, as my own emphasis is on working with preferred carry guns, to see how they do.