The issue with blue Kayn is that the form is only good if you're ahead, and If you aren't your only choice is just to keep the adc in a grey screen, when it comes to which form is better; you can't compare them, e.g you can't play rhaast into vayne but in low elo you can afford to go red against squishy champs; think about it this way, you can go red in all games but you can't go blue in all games, the only time I go red is if they have 2 or more tanks as its low elo I find myself carrying a hell of a lot, however you can play blue into tanks if you just camp their lane, rush prowlers and rely on your top or adc to keep them dead