Get something that is rated for at least 2 million Scoville Heat Units. This is critical to ensure effectiveness.
I prefer a cone-shaped spray rather than a stream. A stream gives about double the range, but the cone shape will ensure that you get some in the attacker's eyes quickly at close range.
I am a big fan of the ASP Key Defender. It is one of the easiest pepper sprays to carry. Mine goes in my back pocket. I bent a clip from a 2-AA mini-Maglight to fit my wife's, so she can clip it to a pocket or inside her waistband. Neither of us attach our keys to the unit.
A big advantage is that it can be used as a pocket stick/Yawara/Kubotan if you run out of spray or don't have time to open the safety. In practice sessions with my wife, we found that we had to plan on pounding on the attacker with the device to create the opportunity to open it. The same could occur with other sprays, but they may not be as well set up for use as an impact weapon.
For people who aren't physically capable of striking, UDAP sprays are a good choice, and also offer the cone-shaped pattern.
Keep in mind that if these ladies aren't 100% committed to their safety, anything that isn't convenient to carry won't be carried. A smaller unit with less spray that is on their person when they get attacked is more valuable than the larger unit that was left home, or carried in the purse that was ripped away from them early in the fight.
