Which has the more powerful Dremora? I have a high level summoner that can bring out a Valkynaz. Are the stats on UESP for all Dremora or just the ones that are enemies?

I think its all. Not positive though. But maybe one of the conjuration perks makes yours stronger than normal ones. This wouldn't apply to the staff however.

the sanguine rose levels with you i believe so basically low levels- conjure dremora lord high levels- sanguine rose but for conjuration once your level 100 you can summon 2 dremora lords so conjuration wins hands down

Daedric artifacts are not leveled.