One thing to keep in mind when comparing the two services is that PrintMojo is screen printing/embroidery, which therefore requires that you prepurchase the inventory you want to sell. Spreadshirt uses, flock, vinyl, and
DTG printing which allows them to "Print on Demand" which allows you to sell with no upfront costs.
I'll leave the other cost/quality questions for someone else

But it should be easy enough to compare costs by signing up for free accounts with both services.