If the 51B is really exceptional for its grade, with both great centering and sharp registration, then I would go with that very rare specimen of the 51B.
If the 51B has the usual eye appeal issues, i.e., blurry and off-centered, I'd say definitely stick with the 52T. That example is nicely centered and will be hard to replace.
Best option, as has been said above, if it is possible snag a 51B and keep both!
