It's about who is taking the risk, with BigCartel you shell out money upfront to buy shirts + print (guessing how many to get in each size, how big an order you need, etc.) then try to sell them.
TeeSpring is kind of like a Kickstarter, you setup a picture of what the T-shirt would look like when printed and then no money changes hands until you hit your "funding level" of a certain number of shirts, and there's no excess inventory because they only print exactly the right number for the sizes that people ordered.
TeeSpring is almost exactly like CafePress or Spreadshirt or Zazzle or Threadless or ... isn't it? Aren't Reddit just moving in to this space (perhaps white-labelling one of these services?). Fair play to them if they broke in to this market (which seems pretty crammed) and took a relatively large slice of the pie.
TeeSpring is kind of like a Kickstarter, you setup a picture of what the T-shirt would look like when printed and then no money changes hands until you hit your "funding level" of a certain number of shirts, and there's no excess inventory because they only print exactly the right number for the sizes that people ordered.
It's a really interesting model.