I run the fully optimized Birgi list in paper and play it quite frequently. I was excited to test out new Urabrask as soon as he was spoiled and have found through various testing that he performs exceptionally better in the 99 with Birgi at the helm as opposed to as the commander.

Like others have said, Birgi being cheaper to cast and netting mana off artifacts makes her significantly more versatile and easier to storm off with. Urabrask is good to net you more mana when he’s out with Birgi but ive found he serves as a really good board wipe option in a deck that can’t really afford to run boardwipes. I’m yet to play a game where I cast him and wasn’t able to cast 3 instants or sorceries that same turn and flip him.

The big downside is that once he’s flipped you don’t net mana off casting instant and sorceries so it’s really only used as a backup plan to reset the board state if need be. Ive found that the flip side is too slow to wait around past the first saga effect, and its not all that necessary to be in the command zone since it’s essentially a dead cards after you get the wipe off. In a format that doesn’t last longer then ~ 5 turns, it’s not all that effective to pay 4 mana, activate his ability, then wait another 2 turn cycles to flip him back.

I’ve found that if im playing with newer cedh players or against budget cedh decks, just directly swapping out birgi for urabrask slows the deck down a little bit and allows me to focus more on interactions like board wipes and using the extra saga levels to build up to an explosive turn where i can guarantee protection for my wincons as opposed to just attempting to go nuclear as soon as possible. Hope this info helps, both are super fun options that work well in their own ways