talk to 20 ppl and you will get 20 different answers. There will be ppl out there that totally hate one or the other. I can tell you I've always been satisfied with the quality of both. Beyond that IKO is generally quite a bit cheaper.
Iko is garbage Certainteed Landmark 30 AR or Tamko Heritage 30 AR. I'm assuming you want a 30 year shingle. I don't know any 20 people who would ever tell you Iko is better then the two i listed above.