>what exactly is the difference between uptown and downtown, if any?

My answer would be that uptown is usually topographically more northern or higher above sea level - which is often the richer area of town over-looking downtown. Downtown on the other hand is, apart from often being the centre of town and thus located in the bottom of the valley (if there is one), is often the central business district.
In general it seems people use these in just as abstract a way as 'I'm going up to Amsterdam' ,or 'I'm popping down to Lake Garda'. I had a theory for a while that men used the 'up to' and 'down to' exclusively according to geographic relation, but alas, men seem to use them pretty carelessly too. The inconsistency really is quite silly; I've been on the phone before with a friend who said 'I'll come up to Munich', and I replied 'No, I'll come up to Cologne!'. Of course, according to height above sea level, he was right, but according to topography, I was