I only know the UP (I ride one since 2018) but they look pretty similar. What I like of my UP is that when I put road wheels and tires, I feel on a road bike. I think the short chain stays of the UP are significantly responsible for this. When I compare the geometry of both bikes, the main difference is the chain stays length (and consequently wheel base). But I never ride a Warbird do I don't know if I would feel the difference of the 1cm longer chain stays of the Warbird.
Hope this helps