Great topic of choice Wilfred!
I have only been able to own 1 of the new horses, while I own quite few of the old ones. I really like the update TLG has give the horse, my favorite part being that it can rear up on its hind legs! This has been a huge plus for posing and creating a sense of action in building a scene. Also they put a little more detail into a smoother shaping of the head and having the front leg raised slightly off the ground.
As to the old horse, it will always be a classic, has great appearance and so forth. I think that whatever the old horse was lacking they made betting the new one except for one thing. This old horse looks way better in my opinion in "horse armor" (if you can call it that!)
It looks less junior-ized and far more imposing in battle. This is the only detriment for the new horse other than that they are both great to me.
There is no clear winner or better horse in my books, I use them both and like both of their appearances. Probably the best thing about them is that they look ok side by side which is a huge plus.
Sir Valiant~