I have to agree with Albervin.
I have had a 21/19 setup on my Rap with std Avons for over 10 years & wouldn't go back to 2"0 unless tyres become unobtainable. Nor would I go for a 21/18 setup, which is probably why it seems up in the air. The prop stands are a bit iffy, but with a Dave Hill centre stand they have become redundant except when servicing the front end.
The 21" tyre also looks much better under the front mudguard.