I know when I eventaully go for a set of uprated wheels i'll definately be going for 16" with a lower profile tyre, say 45.
I'm no expert on it but in theory this should keep the overall diameter about the same, so handling/accelleration should not be negatively affected.
The upsides are that a lower profile tyre should have less sidewall flex & should therefore offer increased/tighter response when cornering. 16"s will also allow you to fit bigger brakes behind the wheel, so another bonus there, then there's the fact that 16's will fill the arches a little better and so will offer better looks.
All down to personal taste, but my opinion is go for 16"s!!