I've got 19" MRR GT1 wheels on! Have a look and decide if 19's or 20's are for you!
20's aint bad at all, I think the profile fits the GS very well, I decided on 19's cause I was concerned with rubbing issues, road quality and ride! I still regret not getting 20's, but I can assure you 19 inch tires are more readily available in my market than 20 inchers! May be different where you live!
Good luck though!