I think there is a slight difference.
But yet they are pretty much the same.
Both of them don't really have a set time of when is later or soon.
If I was talking to you and a phone call came in, I would say "talk to you later" but I wouldn't say "talk to you soon". I think "talk to you soon" is with hope of talking to you again, but "talk to you later" doesn't imply that as much.
Okay to make it short and clear, I would use "talk to you later" short goodbyes, and "talk to you soon" for long goodbyes hoping that it wouldn't be too long, that's why I would use "soon". I hope I didn't confuse you. If you still have questions please ask!