Being one that has bought, installed and used multiple on-line design solutions I can suggest that you first create a list of things that are important to you. Qualities that are important to us may no even apply to someone else. Do you want the system to assist you with back end adminstrative functions, email list, web engine searches, etc. The other key element is what is your target market including demographics (age). This is critical as typically the more flexible and powerful the on-line design tool is the more complex it is to use for the end-user. If your customer gets confused trying to create a shirt they will drop and not complete it. One other consideration is are you ever going to offer other products other then t-shirts?
If you have specific questions feel free to message me -