It depends - 1.3 adds colour depth features and such, but unless you have the sources to output it (nothing supports it yet AFAIK) it won't really make much of a difference. 1.3/1.3a is just future-proofing for features that could be added. 1.2 is perfectly fine.
It's worth noting, however, that if you have an AV amplifier capable of decoding it, HDMI 1.3 supports HD audio.