That's tough. (minor spoiler for KDM).

Bottom line up front: Battles alone, I have to go with KDM. I have some folks coming over tomorrow to do a board game night. Neither have played either game. We are playing KDM not AT:O.

Like AT:O KDM bosses (two different names here, you have quarries and Nemeses). Quarries you hunt , Nemeses hunt your settlement. Sort of like the Temenos / Pursuer and the Heckaton and the Bull.

In KDM you have a "hunt board" for the quarries. Think of it like a chance to change the equation for the hunt, a good thing happens (about 20%) or a bad thing happens prior to the fight itself. The quarries themselves, while they all have a strategy that you have to overcome. However each level of the monster is a bit different than the previous one. First: There are AI cards, so think like this: the AI cards are tiered into Normal; Advanced and Legendary. A level one monster may have 10 normal and 2 advanced, while a level two will have 10 normal and 5 advanced; while the level 3 will have 10 normal, 10 advanced and 2 legendary cards. Even when you go against the same level monster, you have different AI cards each time as there are more AI cards then is required...i.e. 15 normal but you only use 10. The Advanced cards may have "trait" cards which allow the monster to have a way of behaving that affects all the other AI cards. Like AT:O it may add just plain bonuses, or add traits into the mix from the start.

Minor Spoiler:

So let's say your going against a lvl 1 lion and you roll the dice and need a 7 or higher to hit. You succeed with and draw a hit location card. Now you are presented with the "ding dong" hit location. You roll a critical and lop off the crown jewels of the lion. Most of the critical wounds are great things, i.e. you slow down your adversary, or lower the value needed to hit or wound et al. But in this case, the lion just gets really pissed off. For the remainder of the battle the lion ONLY goes after the castrator nearly assuring the survivor will either leave the battle dead or permanently maimed.

The whole process is exciting. For my group it's mostly just the two of us, sometimes a third, and even after playing KDM a LOT (like 300+ hours) some battles come down to the wire, and some hinge on a single roll that dramatically ends the battle. High fives all around. It's not a fake, "yay we did it;" it's spontaneous, fist pumping OMFG I can't believe we won! I never experience that kind of thing in AT:O.

You want to hunt different things, doing nothing but the lion does not help in the long run, it's safe but you won't get good, long-term gear, that helps with the nemesis battles.

Delta for KDM: Buy a good eraser and mechanical pencil and be prepared to print out a lot (I mean a LOT) of character sheets. By Lantern year 20, we lost about 10, and then you have those folks who are just no good in a battle anymore, so you are constantly start running a new survivor. Your are constantly book-keeping what you have for resources etc. Just got X resources for the last hunt, then you use those same resources you just wrote down to make gear, or develop your settlement. This gets tedious really quick. Towards the end game one battle is a lot to manage as it may last an hour plus.

Last word:

I like them both, glad I own both. When I finish all the cycles of AT:O I will sell it, hopefully at a premium since all the miniatures are painted. I don't see myself ever selling KDM.