Is Fritz 15 a better chess software than Deep Fritz 14? I can only afford one. Would Fritz 15 be better to get than Deep Fritz 14? 

I would like to use Fritz to study opening theory.
While I'm not sure what exactly Fritz 15 will offer (because it hasn't been released), but it's almost certain that there will be a "deep" version. Look at this ChessBase online search, so it's pretty safe to assume there will be Deep Fritz 15. My suggestion: wait for the upcoming version.

However, I don't think Deep Fritz 15 will offer you anything that Deep Fritz 14 doesn't. If we're talking about the user-interface experience, there shouldn't be much because ChessBase always ship an almost-identical software every year.

The most obvious change is the engine, unfortunately, this is not very interesting because Fritz is quite far behind Stockfish and Komodo. If you're looking for the strongest possible engine to study opening theory, you might as well download Stockfish and use it with any compatible client. It's free and plays better chess than Fritz.

You'll need to decide what exactly you want: an easy but expensive bundle from ChessBase or the strongest free chess engine.

EDIT (after product release)

It's nice ChessBase finally drops the "deep" term and it makes perfect sense, who is still using 32 bits in 2015? There is no Deep Fritz 15, just Fritz 15.