it depends on what you plan on most of your future projects being.
if you want to do a lot of software then i recommend Blue, if you want to do a lot of games then it seems either LUA (if i understand what LUA is correctly, i havent looked at it in much detail just yet), or a physics pack, if you want to make an mmo (ROFFLE!) then i believe ben is currently working on a multiplayer plugin.
alternatively, you could spend the money on a book teaching C++ and write your own DLL's, a much better long term investment imo. (probably just killed a few peoples businesses - oops)