Horse is faster than the cart, and once the player motor rewrite has been done there will be other differences like jumping, going backwards etc. Just not worth implementing yet since it would mostly get thrown away. There's a whole thread about all this - it was the first thing I added to DFU. Gotta have horses man!
One pretty detail could be changing the point of view/distance when riding a cart but it is more tricky than it appears because it would probably need new graphics.
But It would be a nice detail differenciating much more the horse from the cart. immersion!