Errr 20's more for show, and 19's more for go, is my general opinion.
Its all down to personal taste, I have 20's on mine, and love the look and that they fill the arches, its also lowered on Eibach pro spring kit 30mm, and correct offsets/spacering (et20)
Looks great at a show, meet etc, and with the correct tyres, they perform fine for what I need.
19's are smaller, therefore a bit lighter, and better performance, and also look good, so you get a mixture of both.
And 18's is what the 350 was fitted as standard, and therefore prob lighter and better performing
Although you can get lightweight 19's etc, so all depends on your personal taste mate
And make sure you get good tyres for them
Warning:- Decent 20" tyres can cost £650+
So consider cost of tyres into your decision