what are advantages/disadvantages of picking 19 over 20"

its all personal interest. some people like the look of 19's more than 20's.. and vice versa. With 19's you can get more meaty tires if you have bad roads, 20's will have a lower profile tire and you'll feel the bumps in the road more. 20's will fill up the wheel well with more rim instead of more tire.. there's plenty of different aspects between both sizes.
-GP-

i understand but in terms of performance which is better

If you what performance go with an 18" wheel. The smaller the wheel the less weight and rotational mass, which will also help with braking

The only reason I would do it is weight. 20's look better.

20's also offer a rougher ride and they significantly hurt the performance of the G. 19's are pretty much the perfect size if you consider all factors

yes and no. according to your theory you would get same results with a lighter weight 19" set of wheels.

Tires for 19s seem more expensive IMO

they are...

i say go with 18's or 19's for Performance

^Agreed... in terms of tire cost the difference between 19s and 20s seems to be negligible more often than not. Also, IMO you sacrifice more performance by going from 19 to 20 than you do from 18 to 19 with the G Coupe.

stick with 19s.