I had a chance to try all three at the DOE/NTC SPOTC 2007.
http://www.spotc.doe.gov/video/I liked the M&P45 the best of the bunch. Fit my hand best, handled best for me.
The HK45 was nice, but the grip is too long for me. They squeezed the mag to make it narrower, so now the HK45 10 shot mag is taller than a USP 12 shot mag. The HK45c is about the size of the USP45c and held 8 rounds. HKs LEM is OK, but I would prefer either in DA/SA w safety.
I know there is a difference between the G21 and G21SF, but I can't feel it. Still too thick for me. What they shoulda done is sqeeze the G21 mag so it's a thinner 10 shooter instead of a 13 shooter. That would be better pistol than the G37 GAP.
Liked the M&P in 9/45 better than 40 (I like most guns better in their 9/45 versions than 40). Liked the M&P45 better than the FNP40. They didn't have a FNP45 to shoot, but the grip looks too long for me. Shoulda made it a shorter 10/12 shooter w a mag sleeve for the longer 14 mag if you want more. Kinda like SA did w the XD45 compact frame (another good gun).
Now I have two of my own M&P45s; both black, one w safety and one w/o safety.
Also got to try the HK416 (5.56), HK417 (7.62), and HK MP7 (4.6). All very nice.