I've only started skaven recently, but I can say with confidence that the models are fun to paint (i mean the centerpiece of your army, such as the screaming bell, plague furnace, warp lightning cannon and doom wheel). The troops are too numerous to be called "fun"
lol. But still, with their numbers, you have freedom in variation. You can give each rat a different fur color, different spots, etc. Still, there's only so much variation you can do on
DE (hydra, cold ones, etc) compared to skaven in terms of painting and converting.
Speaking of converting, skaven has a lot of units which doesn't have an actual model. (Hellpit abomination is a prime example). You'll have a lot of fun making your own abominations with putty and clanrats / rat ogre parts

So, bottomline : Go SKAVEN!