I like both, but actually prefer
Theodosia, It's just so cool. I can kind of see the elements
Teddy and Ursa, both meaning bear. So I just look at this name and see a big, adorable teddy bear lol. But it just sounds beautiful. The only reason I would use it is because I couldn't really see myself personally using any of the nicknames. Though there are a lot of nice ones, like
Thea,
Tia, Dosy,
Dot,
Dotty,
Teddy,
Theda,
Terry, Tedda,
Tess,
Tessa. There are some good options in there.