HDR10 == HDR400. But this isn't "true" HDR in my opinion. True HDR is achieved with Mini-LED displays (Pro XDR, Dell UP3221Q...) since the local dimming zones allow for higher contrasts. This is just marketing on standard LED displays.
All are mainly the same, but I'd take the UN since it's the newest model.
Dell has a better warranty, build quality and ergonomics, but has higher price tag.