In many respects, the KRISS is closer to an AR15 than the 15-22.
KRISS pros/15-22 cons:
Magpul pistol grips with beaver tails fit on the KRISS without the annoying gap. The trigger guard can be replaced on the KRISS with AR15 drop-in style parts. Replacement triggers can be dropped into the lower with regular length pins and anti-walk pins. The KRISS safety selector can be swapped-out with standard drop-in parts. The lower will accommodate a Magpul BAD lever without needing to be altered in any way. The charging handle on the KRISS can be replaced with standard AR15 parts. The hand guard on the KRISS is longer allowing you to install 'all the toys' to replicate your centrefire AR15, or if you prefer the 'C' clamp support grip. The hand guard on the KRISS is stiffer, so forward mounted PEQ boxes (and similar) retain their zero. The buffer tube on the KRISS is aluminium, and the buffer and crown area don't sag like they do on the 15-22 after extended use. The KRISS accommodates aftermarket 10/22 barrels.
15-22 pros/KRISS cons
The KRISS only has 15rnd magazines. The KRISS magazines lack the follower button, so they aren't as easy to load as the 15-22 mags. The KRISS doesn't feed as reliably as the 15-22 (once the 15-22s magazines have been broken in). There's no way to see how many rounds are left in the KRISS magazine. The polymer of the 15-22 is a lot harder harder wearing (cosmetically) than the aluminium of the KRISS, which looks worn after a couple of sessions –particularly the painted FDE version which starts to chip the very first time you use the charging handle, and the very first time the bolt cover snaps open. The 15-22 either has moulded picatinny rails or M-LOK slots, whereas both the STD and LVOA KRISS models require proprietary interface picatinny rail sections. The flash hider/muzzle break on the 15-22 is fairly standard, whereas the the KRISS has a proprietary design that is very difficult, but not impossible, to remove. The 15-22 is far lighter, and therefore more pointable. The cleaning interval on the 15-22 is far, far, longer than that of the KRISS. After a couple of mags (30rnds) the KRISS feels decidedly 'gritty' regardless of the ammunition used.
I currently have an original 15-22 (quad rail), a STD KRISS and a LVOA KRISS, and I'm trying to decide which of the three I'm going to keep. The LVOA is the most comfortable to shoot, but can't be moderated. The 15-22 is the least comfortable to shoot due to the short hand guard, but it can be moderated. The STD is somewhere in between the two, and it can be moderated. Ultimately the two KRISS models are considerably more expensive than the 15-22, but don't necessarily offer any additional compelling features. I suspect that the KRISS models will last longer and take a heavier beating than the 15-22 (the sagging buffer tube is a bit of a concern), but that the KRISSes will look very tired, very quickly.
At the moment I'm leaning toward keeping the 15-22, based on how reliably it functions.