They are both very informal. Formal would be "Thank you very much." You could also say "I really appreciated that."

Around 999,950 , I would say.

There is no difference between them but the second is usual

informal→thanks a lot formal→thanks a million

I would put the difference between formal and informal as being "thanks" is informal and "thank you" is formal. "Thank you very much" would be the best option for a formal setting, but I agree with whoever said that none of these options are going to offend anyone.