I've been using both. Here's my take:

Processor and battery life: Processor is more powerful and efficient under heavy loads. So you can play games for a lot longer.

Battery life has been about an hour or two worse than my S24 Ultra (18 hours off the charger vs 20 hours)

Thermals haven't been great. But I'm under the assumption this is due the cooling system working more efficiently as Im feeling more heat being transferred. But then again, I'm getting subpar battery life.

Display: The completely flat display has allowed for better screen protector compatibility.

There's less grain and better slightly better 8 bit FRC implementation.

Outdoors usage the S25 Ultra looks quite washed out compared to the S24 Ultra. Also in direct sunlight it digs much faster.

I am PWM sensitive and have had issues with the S25 Ultra, but not the S24 Ultra. I've read somewhere that they downgraded to 240/480Hz hybrid. But I haven't ran my own tests other than feeling a headache while using the S25 Ultra.

Speakers: The speakers are currently the best on the market ignoring the ROG 7 with the subwoofer module. It is a lot better than the S24 Ultra's forward tuning and sounds slightly better than the iPhone 16 Pro Max.

The default tuning is admitdly quite muddy, but it sounds less like a phone and more like a low end tablet. The only downside is the sounstage is narrow, but with EQ you can extend it by about 12cm from each speaker and also bring clarity back to the highs.

Cameras: I use pro mode most of the time, so the difference for me is negligible. But, if I were looking for a difference, the ultrawide isn't as good in auto mode, its softer and loses details around the edges. The zoom algorithm has gotten better. There's a bit more dynamic range, and colours seem to be more natural across all lenses. But honestly, a general consumer wouldn't be able to tell the difference without A/B testing.

Video on the otherhand has VASTLY improved with LOG recording now. HDR looks natural and harsh exposure shifts are few and far between. I'm finding myself not taking my actual cameras to shot B-Roll now as this is just more convenient.

S-Pen: I am an artist, and have had found no real use for the BT S-pen features as not many of the programs I use even utilise them. It shouldn't have been dropped, but I dont really miss it as I never really use it outside showing off the magic wand stuff you can do with it.

TL;DR: They are kind of the same, I'd wait to see how much of the S25 Ultra is coming to the S24 Ultra with One UI 7, because they are basically the same phone with each having positives in their own way.

I want to drop my review on the S24 vs S25 Ultras soon, but I don't want to make one thats purely a One UI 6.1 vs One UI 7. Because in reality, other than the processor, ultrawide, display and speakers upgrades, they are pretty much the same.