Opto tends to be smoother (as it is governed by photo-electric cells - well, in hardware anyway) with a more... umm... "human" and "musical" characteristic. A regular VCA acts immediately as voltage is applied. An opto has a little "wiggle room" built in. Absolutely wonderful for some stuff, I would argue "most" stuff, others would disagree and tell me that I'm insane and should be lokced away.
Most also tend to be a bit "warmer" sounding. Great for taking the edge off really breathy voices for example, but you wouldn't want to use it on a non-breathy voice if you were trying to make it sound breathy...
Did that make sense? It's been a long day...