There is no real difference between the two as the meaning can be understood as the same.
‘Speak to you later’ could mean you’ll speak to the same person again later on in the same day.
Whereas ‘Talk to you later’ could mean speak to you again in the future some time.
Both can be used to close a conversation.
Note: As with many good questions, like this one, it always generate some degree of opinion based on experience. Bare in mind that the answer has been based on one’s opinion rather than references.
 

My experience is different. I see no difference.
Later = an unspecified future time  (two hours from now, tomorrow, in a week/month/year, etc.). 
The use of speak or talk does not affect the meaning of later.  
There are many ways to say bye casually/ playfully:
Later!, Catch you later, See you later, Smell you later! (a comedian in every group), See you later alligator… 🙂
(The response to the last one is in a while crocodile. :P)