This depends on what 3 and 6 you're looking at. A 6i doesn't have any more power than a 3S. A 3i is the least powerful, but only by a few hp. I prefer the 6 because for me, it's the perfect size. Room for 4-5 adults but not a tank. My 02 Altima was too big, my 3 was too small, but my 6 is perfect.
I was impressed with the 3's nimblness when I bought it, and it got even better when I upgraded wheels and tires. It cornered like it was on rails with 225 series tires on 17" wheels. It was a good looking car, but ended up being a little too small for what I needed, although it has more room than some of it's competition.
I think the 6i has a ride / handling characteristic that leans towards the soft side for comfort, although the ones I've driven handled fine. I've never driven a 6S. My MS6 handles wonderfully, but does have a choppy ride.