Chris B You know the real answer is both... 13" for your track days and 15" for all other activities. I went for the 15" wheels on my 270R (nowhere near as powerful or as much fun on track as your 420R will be I'm sure) and got myself a set of second hand 13" wheels (6" front, 8" rear) that I am more comfortable getting marked, chipped, scuffed, pebble dashed when on track. Allows you some slightly more aggressive tyre choices also for your track days. I too think the Orcus 15" look better but also agree that the wider rears on 13" look good. In an ideal world purely from a looks perspective would be some 15" Orcus alloys that are wider at the rear - similar to the 15" multi-spoke wheels that the CSR variants came with. Have a great time.