I agree. There is no difference, only for the sake of formality, some people say "I'm certain" to stress that they have no doubt about it. But then the most commonly used is "I'm sure" so it will depend on who you are talking to or how you want yourself to sound like.

No difference.

I think its more of the context that the two are used. The term "I'm sure" is more weak, in a matter of speaking. Its like saying "I'm sure I left the car keys on the table, but I can't find them". It's as if you're justifying to yourself that to your knowledge this event / subject existed / happened. Although, using the term "I'm certain" in this situation would not be wrong. It would still make sense. Personally though I think the term "I'm certain" has more weight behind it. Its more of a definite statement. "I'm certain of this....". Overall, I'd say you can get away with using them interchangeably.

i guess there s no difference."i m certain" is more formal ,so it s used less than '"i m sure"