Which particular models do you have in mind? I assume with 2k you mean 1440p? In my opinion 28" is on the small side for 4k (eventhough there exists at least one 24" 4k model right now as well), you'll need to scaling for some applications. Sometimes that works well, sometimes not so well. Windows 10 is said to improve scaling, but that remains to be seen. 1440p is a sweet spot for 27" in my opinion. It looks razor sharp, and gives enough screen real estate without text being tiny. Keep in mind that you need more graphics card power to run higher resolutions as well.

I can't comment on how well VSL runs on different resoltutions. It probably would be best if you could compare your models side by side in a store and see which one you like better.