I don't know who to buy

The anserw is always blitz

sej is better overall, and has a more common playstyle.

sej is probably going to be the most "rewarding" and fits in to a typical teamcomp better and yorick is getting his whole kit changed if im not mistaken (not soon but sometime)

also yoricks kit can be confusing if you are a newer player id say try him in a free week first before you commit