The question of which is better is relative in this case, as both Heat-sinks have properties that make them unique from one another. Both are equivalent in temperature, however thats where the similarities end.
The VX can take a wider variety of fans and does not come with a stock fan. With a standard 1500-1800 RPM fan, the temps are similar to that of a D-14, and adding a second fan in push/pull configuration will yeild a couple of degrees drop. Two 120X38mm Fans will make the Venomous X out-perform the D-14.
The D-14 on the other hand is more of an all-in-one HSF. It comes with two nice Noctua fans, which are very quiet. Also, you will be limited to different fan configurations, but realistically, if you buy this, then thats not an issue for you. It is also more expensive, but then if you bought the VX you would have to calculate an additional 10-30 dollars depending on your fan preference. The biggest thing that sets this apart from the VX is the D-14s size. You will need to have plenty of room to install this beast.
On both you will need to remove your motherboard at least enough to gain access to the backside for installation of the back plate. It is well worth the time, and is also one of the reasons it performs better than other heatsinks.
I hope this helped.
