I prefer DeChlor. All it does is dechlorinate, but you use a couple of drops per gallon, and it lasts forever. Prime is nice to have around, though, if you have an ammonia or nitrite spike, or when you get new fish shipped in (a dash of prime in the bag when you first open it increases survivability rates tremendously).
All dechlorinators (afaik) use sodium thiosulphate for dechlorination. They all do the same job on chlorine, the difference between them is the addition of other agents to either detoxify ammonia or add stress coat. The additives dilute the sodium thiosulphate, so you use more per dose with the high end dechlorinators than you do with the cheap ones. If you have lots of tanks and money is an issue, go with a cheap one. If you can afford it, go with Prime. I buy prime in the big bottle made for ponds.