As already mentioned, they are truly two entirely different firearms. If possible, it would behoove you to handle each and would be even better to live fire each.
The fit of the firearm in your hand is of the highest consideration. Something that feels great in the store is not always so great on the firing line.
I own an M&P, a 6906 and a 3913 and love each of them dearly. The M&P has the interchangable backstrap and usually can be made to fit almost anyone. S&W also has a Magazine promo where you can get 2 more magazines for it.
For myself my 3913 spends more time on me than the others simply due to it's very thin profile and I LIKE how it fits my hands as well as how incredibly accurate and reliable it is. You will see on here that the 3913 has a VERY strong following for good reasons. I, like many others feel that it may be the ultimate CCW handgun. Many have tried to upstage it, however few have achieved that. You CANNOT go wrong with any of your selections however. Fit is SO important...much like buying a pair of shoes. We don't buy size 9 shoes if we have a size 7 or 11 foot.
If you can get to some of the larger ranges where they have rental guns I would encourage you to rent them and see for yourself. Or maybe a friend or two will have some of these to try as well.
As an instructor for over 20 years I see students all of the time using firearms that do NOT fit them well and then they wonder why they are not doing well.
Hope this helps you out in some manner.....
Randy