One difference that I notice that no one has mentioned yet is that Americans like to "go ahead" and do something.
Example: "So now I'm gonna go ahead and put the cake in the oven"
Example: "So now I'm gonna go ahead and put the cake in the oven"
I'm going to go ahead and do a thing - can be used for any time (eg as a confirmation, I'm going to go ahead and mow the lawn (referring to later today at 4pm, confirming intent)). I'm fixin to do a thing - is more often a delayed action set into the future (by at least several minutes) and can be used for any longer timeframe as well, and is often less definite of a statement (fixin usually means I'm going to attempt to, if I can get around to it; or I'm going to try to get to it).