The best answer I think anyone can give is to simple chose whoever you like better both sides have something going for them, personaly I would say that Empire would be propobly better choice for a orc, as there is some backstory to orcs in legion, and if I remember correctly majority of orc strongholds are in empire controloed holds. Even the whole Dominion thing it's made pretty clear that legion knows perfectly well that this is temporary peace to regain strengh before next great war.
That said Stormcloaks also can be a valid choice for orc with little of roleplaying( thinking) .
As I said at the begining choose the side that you prefer you can switch sides after the first mission , so just try.
Neither at this point. The Empire is too weak and timid to get Hammerfell and High Rock to allow the orsimer to return to their previous lands given to them by Uriel Septim by the end of TES2, and while the Stormcloaks don't seem to mind the strongholds, I highly doubt Ulfric will use an independent Skyrim to push Hammerfell and High Rock around just for the orsimer to have their own home.
The way I see it, it's going to be a few orsimer lifetimes before they'll be able to return to their former lands, no matter which side you join.