Depends on what you want to do and how much 3D programming experience you have.
If you are new to 3D and/or programming then 3D Classic is cheaper and although older and lacking in a lot of DBPro's more advanced features, it's ideal to start with. There are also upgrade paths to DBP when you outgrow it.
I've got both and I've not yet found I've needed anything that DBC didn't provide. Having said that, I am using the Enhanced version of DBC and without that, I would probably have switched to Pro a good while back.
DBC, being older is inherently more stable than DBP, but any bugs there are in it will not be fixed - ever!
DBP on the other hand is still being developed and as you can see by a quick scan of the Bugs forum, it's still can't be classed as stable yet.
Both DBC and DBP have the same basic command set and more-or-less anything you can produce with one, you can with the other, though the DBP version will be faster.
I suggest you download the trial versions of both and have a play.
So, your choice.
TDK_Man