Which is better for Katarina? They seem quite similar and both useful on katarina.

It's simple really
You build both

imo you build abyssal against a burst mage and haunting guise otherwise
but i dont play kat so i wouldnt know

I build Sorc boots, Haunting guise and Abyssal in that order. Your abilities do true damage to most enemies at this point, pretty scary.

Once you have Sorc boots and Guise you should be sittin' pretty. That being said, buy Abyssal and Void Staff

People actually build haunting guise?

^ Yes we do, especially since a Red made a post making a really good argument for it.

As Kat was my main before the relaunch I've messed around with a lot of builds since then, and I've found that I get my best early game damage when I rush Sorc Shoes and Haunting Guise. If I'm up against a really bursty mid (like LeBlanc as people have been saying) then I go for a Negatron and rush Abyssal along with my boots. Usually though, you won't really need Abyssal until later in the game when the enemy actually starts building MR but if they full-on stack it you'd be better off with a Void Staff.

You will probably sell the Haunting Guise later on, but I would almost always take it for the Laning Phase at least.