I'm a huge fan of 21:9, and own the Samsung. I have never used a 4K monitor, so cannot comment.
Dell:
Pros
- Great colours
- Decent pixel response rate
- Great build quality
Cons
- Backlight bleed - I can almost guarantee it will have it, I purchased and returned THREE of these (2 x A00 and 1 x A01 revs) because it was driving me crazy in any scenario with relatively dark screen (non 21:9 format movies, games like Thief, Wolfenstein, Metro, etc. made me feel like I was looking through a dirty pane of glass in all 4 corners).
Samsung:
Pros
- No backlight bleed
- Also great build quality
- Slightly prettier than the Dell
- No "black crush" problems that some people say VA panels have
Cons
- Colour dithering (you can more easily see the transition between bands of colour, they go on about it in the Samsung S34E790C thread with some good examples)
- Pixel response time (using the monitor OSD panel 'Standard' setting is a blurry in my opinion, 'Faster' is pretty good but not quite IPS level, and 'Fastest' is pretty good, but overpowers the pixel transitions leaving ugly dark blurs behind some screen movements which is not acceptable).
If you dont mind backlight bleed the Dell is perfect, bleed is really the only caveat. Unfortunately, it is a big one, and I found it totally unacceptable.
I went with the Samsung, and think it's the better product taking the above into consideration. If you can get it for such a cheap price, it seems like a pretty obvious choice. The only thing that bugs me is the pixel response rate.