Yes I can tell you that you do have to upscale with 27" 4k and won't get the full benefit of it
But one concern which people have to really think about is the space you have between your eyes and monitor. I already have a pretty ****ed up eyes, and from my experience with the space I have a 27" really is the sweetest spot. I was also planning to get a 34", but again with that amount of space I'm pretty sure my eyes will get tired easily to move from corner to corner of a 34"
This is mainly the reason why I go with 4k 27", I personally believe that 4k is the future. But I won't deny that 34" will really improve you productivity I will surely use it in my office one day
But at home I mainly browsing, youtubing and games
upscaling while browsing won't be a problem, some youtube content is already 4k and surely 4k gaming isn't bad either\
besides I don't like the idea of having black bars while watching youtube or some aspect ration movie on a 3440x1440
again, it's personal taste regarding size and resolutions
but you get to choose good product, value for money and good warranty for each option
at first I wanna get Acer x34, but that price is just non-sense since I've other better priorities to shop
with that said, you won't go wrong with that U3415w