| Basically what Interludio44. The only real plus sides with bigger wheels per se is you can fit bigger, better brakes under them and if you live in a place where roads are super smooth and flat you can get more grip and control. If you go 17", in fact whatever size wheel you chose, the lighter the better.
Downsides are decreased acceleration, deceleration, worse ride comfort due to smaller sidewalls, less flex so less "give" and feel when cornering on uneven or "close to the edge" roads, increased road noise (often) due to wider wheels and tyres, more expensive tyres.
I can't personally see any good reason to fit 17" wheels unless you plan to fit big brakes or they came with the car you specced already. If I was going to buy new 17" wheels then I'd go aftermarket for some nice light rims, unsprung weight makes such a huge difference to the character of the car.
There's always pros and cons, but I feel with big wheels, unless you're careful AND have a powerful MINI, the most common benefit is a perceived improvement in looks and style. | 
