I ordered my Car with 18" but one showed up down the street with 20" and I had a hard time buying it cuz I really wanted to 18"s
in the end I canceled my order and bought the 20"er one and planned on putting 18's on it still.
But I must say after the first week of driving it, I loved the 20's and when I see a 18" R/T I think it doesn't have near the impact at all when they roll up or sit next to mine.

go 20 if you can, but I'd suggest dropping it 1" so it looks closer to the SRT