As for me I'm considering racing again (haven't since the 80s!) and I've yet to decide on 20 or 24. It appears that alot of guys do both just to get more time on the track.
I've noticed quite a surge in popularity of retro styled cruisers (on Youtube) but was informed in a different post that those retro bikes aren't like the race bikes of today.
I'll be able to consider the options more clearly when the track opens up again and I can go see for myself what the old guys are up to.