5020 all the way, as the brightness isn't much different, but the contrast and black levels are far superior on the 5020. Even if you have a room without sufficient light control, you'll still appreciate better blacks as soon as the lights go down. If the 2100 was a light cannon, I'd ask about your room setup and how you'll use the projector (gaming, movies, tv).
PS: I've used cheaper aftermarket bulbs on this projector with great success. My first 2 Epson bulbs lasted 800-1200 hours apiece before blowing, but the cheapo one I got on eBay is on 3300 hours.
You can use the 5020 for a few years until 4K is better/cheaper and content catches up.