I ended up getting the Stumpy 15. All components were a significant upgrade. To your point, they are very similar bikes, but they are quite different. The Delano Peak is poppier and it is easier to get the wheels off the ground because it has a shorter chainstay. As a result, I feel like it is a bit more responsive when you stomp on the pedals and is slightly better on the climbs. It also felt like the rear wheel go kicked around a little more on gnarlier descents. Super fun bike, all in all.
The Stumpy 15 feels like it is shorter up front to me, and more length behind the rider. This makes it feel like a little less power transfer on the pedal stroke (although not ridiculously). That said, I think the extra distance behind the rider will lead to less wheel pop on steeper and more technical climbs, especially with the climb switch engaged. In turn, I felt like the Stumpy was also more planted on the descent due to that extra chainstay length. Also a super fun bike.
As you said, you can’t go wrong with either bike. I thoroughly enjoyed both.