"I am going to bed" vs. "I will be going to bed"

Both are fluent and natural, but they have different meanings.

I am... means that not only have you made the decision to go to bed imminently or get off the bus, but you are actively doing that — finishing off a drink or TV programme, or moving towards the bus doors.

I will be... means that you have made the decision to go to bed or get off the bus soon but you haven't actually done anything about it yet.

The import of each sentence is implicit in the tense used. I am is present tense and you are actually in the process of doing the action; I will be is future tense and although you know you will be doing something, you haven't started yet.

Your question asks which is more fluent and natural. As a native speaker (British), I would suggest that will sounds rather pedantic despite being technically correct. In most states of imminence I would normally expect am to be used.