you can jump on xerath's face and murder him
that is a lot harder to do to viktor
Both are immobile and are very soft.
In fact Viktor has
less health than Xerath. [516~1842 vs Xerath's 514~1874]; although Viktor has more Armor; most burst is AP-based.
And Gravity Well isn't enough to stop you getting 100-0'ed; especially when you have less range than Xerath anyway and thus are in more danger.
It is just as easy if not easier to jump on Viktor and murder him. The only thing which could save Viktor when Xerath would die is his Q; but then you remember Xerath has an
instant stun; so if Viktor is getting his Q off it's safe to say Xerath is getting his E off.
Both are very vulnerable to being jumped on. Hard to say which one is more vulnerable for sure; but just see how terrified Viktor was of being jumped on by Lissandra.