just adding some meat to the bones here's some questions to ask yourself about each script...
1. does the software do what you want it to?
2. can you understand how to use it?
3. can you extend the features easily at a later date?
4. does the ownership cost fall into your budget?
5. is it customisable, blend into your own site
6. is it user friendly, will your customers understand it?
7. how good is the company support if you need it?
... if you find one that meets all your requirements that is the one to go for