Payton seems to be a forgettable player, so you might be able to buy-low on him. Is sending Harris for him buying low? I don't think Harris has hit his stride yet, and he's still a top 50 value right now. The boost in assists is pretty big...but the addition of more turnovers from a lower-end guy is something to consider too.
I think I've lost turnovers most weeks anyway. So, if I don't care about TO's is Payton the better guy to own?
Harris isn't really buying low, who else could you float over? As you mentioned Harris is already in the top 50 and seems likely to climb from there. The only category where Payton has a real advantage is assists, so if you can try and get him for a 2-1 package that may be better than Harris.
Agree with Hank -- you should be able to get Payton for less than Harris.