If you're wanting differences -
Horse traders plays any 3 cards for 3 coins - baron plays 2 specific cards for 4 coins
Horse traders good at producing 5 value hands - baron good at producing 6 and 7 value hands
Horse traders excellent against most attacks - baron poor against most attacks
Horse traders keeps value as a deck degrades and expands - baron loses value as a deck expands
Horse traders is very hard to improve when the deck improves - baron can be improved as the deck improves (by reliable drawing)
Horse traders hardly improves by hand management (warehousing) - baron improved greatly by hand management
In fact I played a game tonight where they came head-to-head tonight. I went for a potion and familiars, followed by two horse traders. This was a strategy based on attack and defence. My opponent took a baron looking for an early forge, which he got, and just one familiar afterwards. This was a colony game so both our decks were actually in poor shape. My horse traders were always going to be poor at producing platinum buying hands. My opponent was going to struggle with a curse ridden deck and so forged out the baron, who had done his job, and wisely bought horse traders. All our cards (familiars, barons, horse traders, copper, curses, estates) ideally needed forging into something new.