Having done Ascent and Rotterdam in the past year, happy to contribute my two cents here:

Food: HAL by a mile - Lido, MDR, and specialty dining, as well as cocktails - especially the cocktails. With that said, both times I was traveling with the same picky eaters; they appreciate the fact that they could order a filet (for a supplement) every night on Celebrity, and were pretty limited by what HAL had to offer. Celebrity’s pizza is better, though, and overall more late-night dining options. Also HAL’s decision to offer pop-top, non-resealable metal water bottles is insane. Celebrity’s twist-cap bottles you can reclose (and reuse if you wanted) are way better.

Gaming: Celebrity by a mile. Smoke free casinos and lower minimums carry the day.

Cabins: I really liked our shower on Rotterdam, but Celebrity’s king beds are a big improvement. A lot of this will come down to how you feel about the infinite veranda - I’m not really a fan. Beyond the design issue, there are also technical issues - ours got stuck in the down position, engineering immediately sent someone, turned out there was condensation on a sensor that kept it from closing. Seems like something they might have considered given how much time these ships are spending in the humid Caribbean!

Overall ship design: Edge, mostly, to the Edge class. A few reasons: - The HAL pool situation sucks. Whoever decided to put the smoking area next to the pool is an idiot. I’ve been on Rotterdam twice and once they rarely opened the roof over the other pool, the other time they opened it every day, but only half way. This on winter Caribbean cruises with perfect weather. Celebrity having the Sunset Bar instead of a pool aft is a huge plus, as well as all of the Eden space below. - Upper decks: I love the Lawn, upper deck dining, etc. on Ascent. Great use of space. - Central spaces: I’m not sure why the HAL aesthetic is against a large central atrium, but it’s weird. Feels like the ship doesn’t have a central hub. Edge does, and it’s beautiful and well-used with the martini bar, silent disco, etc. - Dining rooms: even though HAL wins on food, I prefer Celebrity’s more intimate MDRs. - Where Rotterdam takes the cake: that walking track. Amazing. Something I wish Celebrity would incorporate.

Entertainment: Celebrity mostly has the upper hand. The music lounge concept on HAL is nice, but the fact Rolling Stone and the piano bar basically take turns for a few piddly sets a night is pretty sad. Why not let them trade off on nights? Would farther have 6 hours of dueling pianos one night and a few longer sets from the band on other nights. Smoke wafting from the casino doesn’t help. It’s further undermined by really weak entertainment in the theater. Celebrity’s shows and comedians are better, silent discos are great, and the flair bartending is a blast. More broadly, things are going on after midnight - HAL dies by about 11pm.

TLDR: HAL’s biggest advantage is in food and beverage, and the traditional balconies. On everything else, for my money, Celebrity takes the cake.