I would say it's just for looks and personal preference. I have the polished Injen CAI and love the way it looks. I'm not sure on this and would like someone to correct me if I'm wrong but the polished one might conduct heat more.
i just got the poliched AEM cAI. its just for looks, in terms of aem vs injen, injen does not powder coat thier intake and will retain slightly more heat than aem (aem clear powdercoats thier polished CAI.) But all in all, the polished application is just for show.....no addded performance. it looks crucial tho. what makes a good intake? ummm, sorry guys, but the piping diameter and filter, they will all claim HP upgrades, but at the wheel, the difference will be like one or two horses tops. seriously, the power will be about the same.....its the quality of the product you should dish out $ for.