I use both 15 & 16. Brand new and used.
Used to be exclusively 16, but over the years when I've wanted to buy used (for some experiment for which I didn't want to pay $200) it's been mainly 15's that were available. Not quite sure why that is. Does no one trade in 16 lb balls?
Anyway....I find that most of the time, there's not much difference. But I do seem to get away with more bad hits with the 16's. That would be a plus, unless you consider that it could also make me sloppier or lazier in the delivery.
Contrary to most, who advise not to mix and match, I've found that if I start tiring (not so much from the bowling, but from a long day), switching from 16 to 15 gives me a little extra energy and sometimes revives my game. And other times, if I'm overthrowing the ball (too much energy), switching to a 16, helps me get my timing in sync.
Most of the time, I still choose which ball to throw, based on the lane conditions and whether I think it matches up well at that time. The weight is only a secondary consideration.