Cooling performance is really all about surface area, and in this regard it's quite clear the D14 is larger than the H50.
Okay here's the D14: http://i813.photobucket.com/albums/zz58/shawn_sotto/464967_3127937516918_1219102615_32360195_907461070_o.jpg
And here's the H50: http://i173.photobucket.com/albums/w45/hitman1985/Panzerschreck/IMG_3052.jpg
One thing is clear, with one of these coolers installed you CANNOT EVEN SEE YOUR MOTHERBOARD, with the other you can completely see it.
BUT! The Corsair H50 does have things over the D14, one of these being aesthetics. If you would rather have a clean looking all-black cooler rather than a giant brown poo cooler then some may prefer the H50, and not to mention if your RAM dies you need to remove the heatsink to get to the RAM.
For coolers, I would also look at:
Phanteks PH-TC14PE (comes in many colors)
Corsair H60i (the newer i series perform a LOT better due to their vastly superior fans)