Bigger wheels mean more weight, i'm back on 14's they handle loads better (for me) than my wide base 15's but they have great traction.
All depends on your needs, handling, speed, etc
im getting i 1.3 micra, and i just want nice looking wheels that give you a good 0 - 60 speed, because on my cinquecento i put 142s on and it slowed it down alot.