BTW: New Gen 4 Glocks come with interchangeable backstraps for different sized hands, etc. This includes 2 backstraps that have a beavertail. May want to check out someone who has a Glock 17 G4 and have a feel before making your final decision. 
I'm considering the same two while I wait for my PAL/RPAL. I took a handgun level 1 course and got to shoot the Glock 17(G3) and M&P 9. Went through 250 rounds that day and about 100 rounds back-to-back between the both of them. As a new shooter to handguns, I can't say if one was any better than the other one. The M&P did feel more comfortable in my hands though. My hands take a medium glove size. The longer beaver tail on the M&P 9 made it feel more planted in my hand. Not sure what backstrap size was on it, but the interchangeability is what will probably sway me towards an M&P. Cheers.