The slide release on my M&P40 was stiff as well. It got easier to push as I handled the pistol more. I asked some family members with M&Ps and they experienced the same this as well.
Personally, I rather get off more shots in a bigger group (4"- 6" inches) quicker over a few shots in a smaller group (2" - 3" inches) at a slower pace. Combat accuracy. Someone is trying to do harm/kill me or others around me and I want to stop the threat as quick as possible.
Get the caliber you feel comfortable with. At the end of the day that is all that matters.
Each caliber is going to protect you and your loved ones when it matters, provided you know how to shoot. The trigger gets pulled, gun goes band and the bad guy falls down....repeat as necessary :)
I...