there called mutants and they aren't symetrical. They are kind of like riding a directional but without the hassles of having to change feet all the time.
A mutant allows you to sail exactly like a TT, or a directional (either gybe the board changing your feet around, or go heal-side to toe-side).
As a beginner don't waste your time with a mutant, just enjoy the ease of your TT.
I guess you are asking as we are in low wind season. Certainly the larger fins os a mutant will perform better. I love my Underground Firebird mutant. It gets up and planes in under 10 Kn, and is great for bottom turns.
a twin tip with large fins on one end is a twintip for waves, you can still ride it both ways but works better one direction. You'll get higher jumps on this then a standard twintip usually.