I would defo play both before deciding. Stingrays have an odd sort of feel to them, I don't like it, it's very chunky and mass-produced-feeling. Thumbs are different again, the position of the top strap point moves the neck a couple of inches further out to the left than you may be used to from Fenders etc. Some people can't get on with that and it's not good if you've got wrist tendon problems, especially not if you're playing fast metal patterns in the lowest positions.
Sound-wise again the Thumb is really quite idiosyncratic. If you want a ton of bottom end you can't have it, but they do make a lovely purry, burpy mid-boosted sound that sits well with distorted guitars. Stingrays are almost the opposite - they tend to sound scooped. Of course both basses have active EQ on board but still they have their own characters.
Personally I really like Thumb basses but I wouldn't recommend buying one without trying one.