I have found no use for the Imperial race because I like to play Magicka (mana) based game play caster and ranged...The horse well you can spend US15 and get the same horse just brown.
The real advantage is you can play any race with any alliance! Which tbh is not a big deal...
The playable Imperial race and the horse are about all that is worth it. Maybe the ring of mara as well if you are playing frequently with a friend. All the editions are negligible of course. You can play as another class, you can save money and get the basic horse around level 35.
Nope you can buy a brown horse from the ESO Store for about 10 quid
I didn't upgrade to it, I preordered imperial edition and no regrets so far. Go for it if you think it worths it. I think it does.