The following are my $.02 cents worth. I recommend to do further research to double check my statements. I am not an expert on this subject.
Supposedly, the higher the concentration, the oilier the mixture of the pepper spray. That means it would be harder to decontaminate. But, I have to wonder if that makes any difference at all. As Morgan pointed out, the true indicator of potency is the Scoville Heat Unit. The higher the hotter. I think 2mil is the legal maximum. That's the one I use.
Streams are perhaps the most common among carry sprays. Unlike cone sprays, they are less likely to be blown back by wind. However, supposedly one does have to aim. They are best suited when outdoors.
Cones used to be the most popular I think. They work more like an area defensive weapon. Llike shotguns, requires less aiming to get the desired effect, but can easily be blown away or even back by the wind. Larger versions of this are available, designed to flood an entire room full of spray, like a grenade I suppose. Cone-like sprays are best used indoors.
Foams bear the advantage of being pretty much immune from wind AND when spray about the eyes, can make sure the spray blinds the assailant, that is unless they somehow wipe it off and manages to see through it. The disadvantage here is that the foam can be thrown back at the sprayer.
I admit being personally biased against foam. I don't see what the foam can do that streams can't. But some people swear by them.
Perhaps the most important feature of any pepper spray is the container design that will allow your wife to carry one. I have a female friend who's uniform she wears to work forbids the use of most sprays. Some containers are more practical than others. For example, I love the ones with a spring top lid, finger index and integrated clip. It is much better than those small, cheap leather covered things that dangle off of keychains. But again, it's a matter of finding out what your wife can carry and what she can't.