I bought the 3 Pack of 17 round mags. At the time, nobody had 15 round mags, and the price for the 3 pack was pretty darned good.

As far as feel or balance, there really isn't a lot of difference. And two extra rounds for a lower price seemed to be a good deal.

If it ever happens, nobody knows ahead of time how many rounds will be needed. I'm an old timer, but a guy older than me said it was "One more than you need."