Buying my first handgun here in a few weeks. I have them narrowed down to the glock or the M&P. Which is better? Which one is better for a rookie handgunner?
Neither one is better - they are different. I happen to prefer the M&P, but that doesn't mean a thing, nor should what anyone else prefers.
They have very different grips and very different triggers. What you need to do is - ideally - shoot both and decide which one feels better in your hand, which one feels like it has a better trigger pull. What feels great to one guy just doesn't feel quite right to the next. At the very least try and handle both side by side first.
The M&P does have interchangeable grip sizes which can help to get that "just right" feel, but it doesn't change the grip angle which is where many guys base their likes and dislikes. They have very different triggers; I think the M&P comes closest to a single action pistol's trigger, but you should decide which one you like the best, not what somebody else thinks.
M&P's have no warnings about shooting lead bullets through them. As a reloader, I prefer shooting the cheaper lead stuff to jacketed for practice, whether they're bullets I cast or bullets I bought. You can buy an aftermarket barrel for Glocks of course, but then you need to figure that into your purchase price.
On the other side of the coin, you can buy a .22 conversion kit for the Glock and, as far as I know so far, nobody has released one for the M&P yet. I have a conversion kit for my BHP, and I think they're a nice thing to have for a centerfire pistol - even more trigger time for cheaper.
As for caliber, if all you're shooting is paper, I'm at a loss as to why you would want a .40 over 9mm. I guess you'll get a few more biters now and then in competition, but I'm not sure I'd exchange that for the stronger recoil and increased reloading costs. I am very happy with the .40 in my BHP, but then I carry that handgun for self defense, not punching paper. If it was all about recreational/competitive shooting, I'd sell all my .40 S&W stuff in a heartbeat and replace it with 9mm.
I'll bet I'm not the only guy who suggested trying them both and selecting the one that fit YOU best... right?