Well, the 3770k has better instructions per cycle, but has a thermal paste applied heat spreader, so isn't quite as good at overclocking.
3930k has more cores, more cache, a soldered heat spreader. Not sure I would want an old or used water cooler, likely out of warranty. Air coolers are easy to deal with. Though the cost difference is certainly reflected in the two prices.
Are you planning on overclocking either? Doing any video streaming (like Twitch) or video encoding?
3930k = 6 core Sandy Bridge-E + quad channel DDR3 (130W)
3770k = 4 core Ivy Bridge + dual channel DDR3 (77W)