Hey! sorry for the super late reply.
Dark Harvest can sometimes work on Xerath (same for other long range poke champs like Ziggs, Lux or Vel'Koz. It can be useful if you know that youre going to get alot of stacks. If you dont get any stacks its going to be pretty much useless.

Arcane Comet adds additional poke to your spells, helping you in lane mostly, but also in teamfights. First Strike gives you a bit more damage and bonus gold income, making you reach your powerspikes faster.

If you take Dark Harvest instead of those 2 runes you trade in bonus lane poke (or bonus gold) for a chance of higher damage later into the game. It will definitly be better in teamfights when you can get good hits and stacks - but if you lose teamfights and the enemys dont get low its useless.

So basically its like a double edged sword - can be better, but can also be worse. You never know - thats why you take Arcane Comet or First Strike - because they will always be good.

ok sorry for the giga late reply xD
Sylas matchup is somewhat even, you normally fall behind early game since Xeraths early is weak.
Once you have Lost Chapter it begins to even out a bit and you can start poking him, but if you get too close he will still kill you

Hey, sorry for late reply! good question tho.

Transcendence is also a good way to go tbh. When u have Ludens + Zhonyas or only the Codex + blue buff you should be at 50% CDR which gives you +20AP.
Compared to Absolute Focus you get ~20AP at lvl 12.

Now the thing is youre not gonna buy more CDR since you need Raba + Void in most games. Even going Zhonyas or Codex 2nd item is a bit awkward since its better to go Raba or Oblivion Orb.

That being said there isnt much of a difference between the 2 runes.
All in all Absolute Focus gives you more AP later on and you can focus on building Mpen or AP.
Transcendence lets you cap at 40% cdr, which is way better then the 30% Absolute Focus when you have blue buff but provides a bit less AP.