Speaking of which....are there any plans for ULTRA HD or 4K mode for large TV screens?
On the Xbox? No. It might support 4K output, but that would still only be via upscaled 1080P, so wouldn't be true 4K.
In answer to the OP, the Xbox One version is still in Game Preview Program, so we've only had two updates, but it's hugely impressive. Framerates are solid, and graphically it's close to Ultra on the PC at 720P. I have a Core i5 system with a GTX560Ti and the Xbox One beats it hands down on the same TV. Very, very smooth. The controller setup is better too.
No doubt this could be read as fanboi-ish, but I'm trying not to venture off topic. Right now I think that Microsoft are pushing WAY harder than Sony. They're focussing on DX12 for the Xbox One soon (I heard August 2015, but that's only giving them a week to get it deployed). The Kinect 2.0 is amazing for gaming and controlling the box. I always thought "Xbox, On!" was a gimmick, but having my Xbox, amplifier and TV all power up perfectly from a voice command is awesome. "Xbox, Volume Up" "Xbox, Go to Forza Motorsport 5" etc. Neat. Really. There are free games with an Xbox Gold membership twice a month. They've varied from indie to AAA. Getting games like Assassin's Creed: Black Flag, or MGS: Ground Zeroes for nothing is more than the annual cost of a Gold Membership, even preowned. Microsoft are also very, very cleverly using the "advance exclusive" system, which Sony seem to have missed out on. For example, Rise Of The Tomb Raider is NOT an Xbox exclusive, but it's being releases a few months earlier on the Xbox than the PS4 or PC. As ever, I'm kind of in awe of Microsoft's evolving marketing strategies.
The PS4 is more powerful graphically, and the API is great for developers, but DX12 gets closer to the GPU metal on ALL DX12 platforms, so conceivably the broader base of DX12 programmers will allow for more long-term optimisations on the Xbox One. It's going to be like the Xbox 360/PS3 comparisons. Ultimately it comes down to the people doing the work and the money invested, not down to the absolute spec of the hardware. I have a feeling that we'll see the PS4 doing things better in some aspects, as they'll be pushing hard for reduced resolution 120fps for Morpheus next year which will have a knock-on effect for 1080P gaming, whilst the Xbox will benefit from a more comfortable development environment. But it WILL be close, and fan boying to one side or other doesn't change what they do best.
Don't turn this into a fan boy war in any case. I've got the Xbox One, and one of my close friends has the PS4. The "Windows 8" front-end of the Xbox One doesn't scale up well, the controller on the PS4 isn't great for big hand etc. They're different machines, so pop into a local Game store (or equivalent if you're not in the UK) when it's quiet, and have a play. They're both awesome, although I honestly think that both Sony and MS could have gone about 50% better for graphics power without breaking their stride. Consoles need to last for 6-8 years in order to "work", so we're stuck with what we have.