Sharper is better. I just booked the last remaining unit of Samsung U28D590D, an Ultra HD, 4K monitor from the local PC store (a display unit) at nice, 320 euros price. This is probably the most budget-consious alternative in 4K, 28″ monitors you can find; there are better, IPS screens (this is a high quality TN), and particularly professional models have better ergonomic in the stand (this is a completely fixed thing, and no VESA mounting either). But the colour reproduction, brightness are excellent, and particularly having 3840 x 2160 resolution at 60Hz, with 1 ms speed (over the Display Port 1.2) makes this pretty much what I have been looking for my gaming and photo editing needs. I am also regularly plugging in several computers (PC/gaming workstation, Macbook Pro Retina, Chromebook) to the same display at my desk, and there is interesting PIP (picture-in-picture) mode in U28D590D where you can keep an eye on the second PC while simultaneously working full screen on the other (let’s see how useful this will be in reality, though). If you think there is a better deal available from somewhere at the 300 euros price range, let me know. More information: http://www.samsung.com/levant/consumer/it/monitor/uhd-monitor/LU28D590DS/ZN.
Edit: this is the thread with instructions for getting 52 Hz at 4k on the retina MacBook Pro 13: http://forums.macrumors.com/threads/4k-display-and-retina-macbook-pro-13.1741440/