I personally think that as a newbie - 9mm is the way to go. There are some 40 cal fanatics - but I am not a fan of the load. I like 9mm and 45 ACP personally. Also, many new shooters jump on a caliber that is too powerful for them, and then they either get rid of the gun, or give up shooting it much.
Plus, 9mm is almost 1/2 the price of a 40, and is 1/2 the price of 45 ACP. I went to all 9mm because the ammo is close to $5 a box. That way, I can afford to shoot 200 rounds every other weekend. I get a lot more practice than I did when I had a 45.
U will undoubtedly get a whole spectrum of advice, however.
Can U go someplace and rent a gun? U can try out different calibers that way - even if it isn't in the M&P.
The M&P looks like a good gun - if I didn't already have 6 polymer guns, I might even buy one 1 day. But it is such a new design - I'd be hesitant to buy one for another year or two - let them work out the kinks...