The only RZT series I would purchase is the 54" model. That one comes with the Hydrogear ZT-2800 transmissions. The lesser models come with the EZT transmissions, which have had lots of problems in the Cubs. However, you would probably be better served moving up to a Cub Z-Force series. In addition to the better transmissions, they have several other improvements that make it a better product, including a much better performing mower deck.
Z-force's are only sold by Cub dealers, btw.
Bad Boys and Grasshoppers are nice products too, but you will pay a lot more for them. If you can swing it, you may be happier in the long run with one of them. If not, then the Z-Force's are probably the best bang for buck out there.