H90 has a larger fan overall (140mm) so that is a good and distinguishing thing.
Its con is that it uses older style cold plate configuration and fan speed can be controlled by motherboard only. Only new technology is the redesigned tubing. It also lacks a few modern features.
But on top of that overall cooling is very good and is not in any way worse than H80i. It also installs very easily. I would recommend it to any normal user who just want to overclock their CPU to a high - extreme level.
I would prefer the H80i. Sure the H90 has a 140mm Thin Rad , but it can't be as good as the 120mm Thick Radiator in the H80i. The radiator ( rad ) is what cools the coolant , and therefore one with more overall area is better ( For e.g a Thick 120mm Radiator can easily beat a thin 140mm Rad , as it's area is more than a thin 140mm one. ) Also the tubing and cold plate isn't good at all. The tubing affects the performance of the flow of coolant from the reservoir ( the place where the coolant is stored ) to the Water block ( the block/devie which cools the part to be cooled ) Plus most case don't have a 140mm mount. On the otherhand the H80i has a much better tubing , better pump , 120mm mount which most cases have , and most importantly Corsair Link support inbuilt , eliminating the use of the other Corsair Link devices to function. The Corsair Link software will help you to adjust the pump speed , know the temps , etc. So my vote goes to the H80i. Also look for other coolers , like the NZXT Kraken X60 if you can get it as its amongst the best CPU Closed LC cooler
These results show that H90 is better than H80 at both high and low RPMs. Note that the H90 also makes less noise due to using only one large fan. I'd recommend using H90 as long as you have space for a 140mm fan. The only argument left for using a H80i is if you're bent on having Corsair Link support.