I own Atlas and I helped with the beta for V2. I am also trialling XO during the sale.
Atlas arguably has fewer features although it works perfectly well for what it does, and at $49 it’s a perfectly decent one-shot drum machine. The sample map is less sophisticated than XO which has extra features for filtering the map by type (kick, snare etc) and other characteristics like length. It has extra features for intelligently swapping sounds and finding new ones as well as for experimenting with changes to patterns.
To be honest, I basically bought Atlas because XO didn’t run well on my old machine whereas it now runs great on my new M1 Mac.
What I will say is this - the Atlas developer was very receptive to my input during the beta and he’s a nice guy. It’s in active development so you can expect new features. People generally seem to suggest that XLN aren’t receptive to feature requests and that the product is what it is with little or no new features since release. This may well be true as Addictive Drums looks identical to when I last looked at it a couple of years ago, although I haven’t checked every detail carefully.
I reckon at half price they are both decent tools that will unlock value from the massive and unwieldy drum sample libraries many of us own. Personally, having had a few days with XO I suspect I’ll buy it at the discount price. It has some minor shortcomings (mainly with the fairly basic sequencer) but it does what it does very well and it’s fun to use and encourages exploration.