It's a pretty complex topic. IMO, at the moment, Mac AV isn't a necessity unless you're either a very naive user and/or you know that your own activity puts you at increased risk. Even then, if you're super retarded then no AV is 100% fail safe anyway, and relying on it may put you at further risk. The savvy MAC user with common sense does not currently need to run AV. Again, only IMO.
Sophos is good if you feel the need, although in my experience is does slightly slow down file access. Although I wasn't rolling with SSD when I used it. If you can live with a potentially slightly slower system, then you won't go far wrong with it.