There are really two factors that you should look at in a throw bag in my opinion. Tensile strength of the rope and length of rope. These are the starting point for picking the one that you need. I like to have spectra rope, I use my throw bags as pieces in my pin kit so I do not have to carry extra rope. I carry two throw bags with spectra bags one is 70' and the other is 55' long and one nylon and 50' long.
70' bit on the long side for rescuing a swimmer, usually hard to hit a swimmer at 70' and if miss and have to do a coil toss it can be tough.
Then other suggestion I would make it to get a waist bag, you always have it with you and you know where it is. It also help if your boat gets pinned and you can't get to your pin kit on the boat.
I always carry a small pin kit on my person just in case. I carry a few beeners, two plastic pullies (Ultralegere Pulley : Rock Climbing Gear : Rockclimbing.com
), and a couple prusik cords.
Hope this help and I also agree you get what you pay for. Skimping on rescue gear?