I would go with the 17" more side wall to protect the rim
I was glad I had 17" wheels when my wife curbed the tires, they definitely saved my wheels. I like the look of both 17" and 20", but any bigger than 20" looks strange IMO. I have the 17" and must say tire prices are still very expensive, I paid over $1000 out the door for my zeon LTZ's (granted they are a great tire). I could have got cheapo tires at one place for like $500 out the door, but as far as normal tires, most were in the price range of about $800-$1100. My boss had better luck as far as pricing with his 18" wheels. I happen to prefer the smaller look of the 17" or 18" over the 20", and as far as weight, I think the extra weight of the wheel balances the less tire, and vice versa, or at least pretty close to doing so. A few pounds aren't very noticeable anyway (my combination is like 15-20lb heavier per wheel/tire over stock, and I can't tell a difference.