The difference is very, very small. My first thought was "no difference." Although I agree with Peachey, no one would notice if you used one instead of the other in casual conversation. Your tone of voice and the context would probably be more important.

"I'm going to meet Ann tomorrow" is your own plan. It's not clear if you've told Ann or not, but at least the plan is clear in your own mind. "I'm meeting Ann tomorrow" means you and Ann have talked, and you have an arrangement: you both know the time and place.

" going to" means plan, without the act of plan, it is more positive. I'm going to meet Ann tomorrow ( I plan to meet Ann tomorrow) I'm meeting Ann tomorrow ( definitely, I will meet Ann tomorrow)