First, you would say “I’m” in both cases. They are different tenses but there isn’t a difference in certainty in my opinion. I’d use the latter if I wanted to excitedly announce a new trip. The former sounds a little more factual. I’m going to Japan next year, but this year we’re focusing on China.

They are identical. An English speaker will say them interchangeably purely based on which comes to their tongue faster.

both mean the same to me

Thank you Chris!

English Grammar in Use 4th edition (Murphy), units 19-22 is excellent. Practical English Usage 3rd edition (Swan), sections 211-218 is excellent. Also, ef.com has a useful overview here: https://www.ef.com/ca/english-resources/english-grammar/future/