hey man welcome to the trials forum....
Really what i say is if you come from a bmxish background go for the 20"
you you ride normal mountainbikes and such go for a 26"
If your fairly small and weak 20 inch bikes are alot easier to throw around and stuff....
Its down to what you really fancy going for.
Its not like mods (20" bikes) are for beginers and stock(26" bikes) are for a higher standard.
Hope this makes sence/helps
Dave