Dave,
Rider numbers is Viper over the years are slowly heading towards pre85 and pre90.
If you were to ride Viper, the RM/N or T would fit into EVO & pre85
VCM still have a large following of pre78 riders so your RM/A or B or the YZ/D would fit right in.
Also a pre78 bike will allow you to ride pre78 classes in Viper, VCM & CSC.
I have a YZ400D which I'm happy with, the pre78 bikes are getting a little thin on the ground but in saying that, you walked past a YZ250D for sale at Barrabool on Sunday.
Between the RM or the YZ I think it will come down to what ever you can get. The YZ would be a bit better as far as parts availability goes.