They are two very different platforms. TeeSpring seems to be adopting some of the features of Zazzle, like having a storefront. TeeSpring sales are time limited models and Zazzle is a cumulative storefront model.
I have a storefront on spreadshirt, what I had planned on doing with it was using a tspring like service to test the designs, if a design did well slight alteration to color or font (keeps the tspring design limited) and then put those on the spreadshirt store. Gives you the best of both.
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