Good question.
I'd do whichever TopoFusion gives you as the default. If the track you are saving has heart rate, cadence or power data with it, TopoFusion will default to .TCX. Otherwise, GPX is the best choice.
GPX does now support HR and CAD, but I'd still recommend TCX if you want to save that data. Not many programs read HR and CAD in GPX (TopoFusion is one of few that do).
So basically GPX is the best unless you have that extra data, and TF will default to that depending on what you're saving.
