Well, I don't think you should make a 29 at this point. Scaling would be better at 25-28, depending on class and spec and what not.
As for 20, I think you should check into the f2p bracket and see the gear scaling list for a general idea of how well it scales. If it doesn't seem to be scaling that much, then I wouldn't stick with it. But at least there you could get an idea and then factor in enchants and BiS gear since you will have access to it all.
However, losing the abilities and glyph could hurt you in the long run... I don't have a p2p 20 so I can't give any more info. Hope this helps a little!