Thresh, Blitz players are usually realy useless but Thresh players are usually good, Thresh has more CC as well and he can do more for the team.
Thresh is more usefull then Blitz. Btw they are so close to be same but Thresh have lantren that can help u so much to escape. So Thresh
Its simple. with blitz u need to pull who ever u hit, so if u pull alistar on some other tank u can get in trouble. With Thresh u can chose will u engage or not, also enemy blitz pull our adc, all what u need to do is toss lantern and pull him back
thresh is much better coz he has way stronger spells tho it aint easy to land the hook like with blitz coz of that delay