It might be worth noting, when investing in an OC Spray product, do some research.
First thing to look at is the reliability of the product, foremost. The leading way to measure the effectiveness of OC Spray is not the Percentage, but the SHU's (Scoville Heat Units). This is a system that's been used to evalute the heat of peppers for years, and it's commonly applied to OC Sprays. The higher the number, the hotter the product. This is a general rule, and there is some controversy over SHU's, but as a general rule, it seems to pretty reasonably equate to the degree of "heat" present in OC. If your OC product does not rate it's SHU's on the can, this might cause some concern about the nature of the product.
An SHU of 2,000,000 would be the least I would carry. One of the most popular brands of law enforcement pepper spray going, First Defense, markets their product at 500,000 SHU, which is extremely mild. They brag that it is food grade pepper, a fact that I can personally attest too as i've eaten it before.
Don't let the idea that lots and lots of police agencies carry it be a selling point. I personally believe that First Defense has a good marketing program aimed at administrators, but that's another story. This product, it has been my experience, is only 60% effective in the field. As evidence that i'm not just talking out of my rear about this number, take this study
http://www.zarc.com/english/other_sprays/oc_strength/firstdefense.html
I've personally had suspects who have been sprayed by this product TELL me the next day (after they sobered up) that it was a liability as it had no appreciable effect. This has resulted in officers being injured because the product did not work as advertised. It is apparent that First Defense has become aware of the short coming of their product, as they have now unvieled a "hotter" brand of their product. Lets hope it's hotter than the first one. If so, the new First Defense product may be a useful contribution to the OC selection.
So, in short, any product 2,000,000 SHU or above is a good start.
I personally carry Fox Labs OC, which has an SHU rating of 5,300,000. I have never had this product fail, and it has the most impressive performance of any OC product i've tested and used. I'd rate the reliability of Fox Labs OC in the field as greater than 90%. Also, if you order directly from Fox Labs, you'll probably deal directly with the owner, who's more than willing to discuss, at length, any issues about his product. You don't find that kind of customer service anywhere. Fox Labs costs a little more than most other products, but it works when you need it.
Other excellent OC Sprays are Zarc, Cap-Stun, Punch, Freeze, Body Guard, and a few others.
There are also a new class of Defense Sprays out, which aren't technically OC, but synthetics. Two examples being PepperBall Brand OC, which utilizes a product called PAVA (Capsaicin II), and a Zarc product called VEXOR, which boasts an extraordinary SHU rating of 15,000,000 SHU! Again, though, these two products are relatively new on the scene, so my department has decided to take a wait and see approach to utilizing them. A neighboring department, however, has claimed extremely favorable performance from the VEXOR product (keep in mind, however, it replaced their long used First Defense stockpiles).
The other considerations when buying an OC product are type of spray (Stream, Cone, Mist/Fog,Foam), this will be dictated by the environment it's likely to be utilized in. If it will likely be outdoors, then the only choice is stream, as the other products are more susceptible to wind conditions.
In addition, size of the container will be a factor. This will be determined by type of carry (key chain, versus purse, versus pouch/duty gear,home use). Key chain size containers are fine if that's all you can carry. I have found that keychain applicators are unreliable. I profer at least Mark III sizes or larger. If home defense is the issue, a fire extinquisher size is PERFECT for hosing down the guy trying to steal your car.
Finally, reliability of the applicator. Some products are more reliable than others.
Hope this is helpful to someone. It represents several years of my experience dealing with these products. It is only my opinion on the matter, so take it for what's it worth.
As a final note, in reference to alcohol based propellents, I believe that it had been removed from nearly all human use solutions of OC. You may still find it occassionally in solutions dedicated toward animal use. The best way of determining if your OC is flammable is to spray it and light it. If it will put out a flame, it's electronic restraint safe.