You can't compare them because they're different types of games and work in different ways, GW is good if you want a teamplay game and a heavy, tournament-styled PvP aspect, WoW is good if you want a larger persistent world, conquest-based PvP and a big time-sink.

Also, GW and D2 are nothing alike because there's no endless repetitive grind for items that are better by 1% or armour that is stronger by one point, it's easy to get a max damage weapon, and it's easy to get the strongest armour and you max out at level 20.

It can be rather hellish to play GW without at least one friend, or a group or guild of friends, I don't mind forming the occasional PUG, but I like to have at least one person I KNOW I can rely on.