Performane, practicality or appearance - what do you want?
Thats the bottom line.
I thought I would loath my cab on anything less than 17's, but was prepared to give it a go, and think it has never looked better. But! and it is a big BUT - it is totally impractical for more than 2 people, it will pick up any rut in the road and track it, and every journey involves great considation because of speedbumps.
15's would be a great everyday practical wheel, and suit the cab too. You can still get it looking low, but it wouldn't have that extra 1/2" crucial drop that you get on the 14's
You are going to get a higher top speed on 17's but it will take a while to haul itself up there, with 14's the take off improves but the top end suffers (110 MPH - meusered on GPS)
There are so many factors to consider, but at the end of the day, it is really down to what you feel comfortable with and are prepared to put up with.