Karthus>TF in solo Q, easier to play, and easier to carry with. Also he scales better with pure ap and he can afford to build tear then rush zhonya. Only downside is he sucks playing against poke comps, but if you get a good speed boost from sivir or an item, you can get a wall down and slow their whole team. Or you can ult before a teamfight since it deals flat damage, and then you have a bit of an advantage.