What ur driving style like?, because there is one thing afffording the rims, the other is affording the rubber... now im running 17's and good rubber is around the $200 - $250 mark for 235/45/r17's , that goes up on the order of about $100 for every inch u increase ur rim diameter, ie 18's is about 300-350 for good rubber.......
thats my 2c, on rubber, back on topic...
18's look nice, this also depends on how low ur car is as well...... 18's are not practical on ym car, as my zorst flange is only 103mm of the ground, and well u know, grinding noise are rather common these days :S
17's are the way to go as far as im concerned, there cheaper, look good, cheaper rubber, and just more practical, if u want to see my car with 17's pm for a pic ok....