Unfortunately Ron is right, the difference between X50 and 996 Turbo is quite marginal, especially below speeds of around 125 mph. The 0-60 time on the X50 is a fraction of a second faster, not even worth mentioning it. Top speed is 2 kph faster, not more because of the shorter 6th gear.
I understand your question and I try to give you an advise:
IF you are SURE that you don't want to have 500+ HP later on (and you REALLY have to be sure), go for the X50. Why ? The X50 is expensive but it is original Porsche stuff, tested to the max and it keeps the Turbo an original Porsche. For people wanting to put 500 HP and more on their Turbo, X50 is pretty useless. The larger turbo chargers are the only thing worth the upgrade but you can get them much cheaper later on. The ECU has to be upgraded anyway, the larger intercoolers are pretty cheap and the so called "enforced" X50 gearbox actually incorporates only a shorter 6th gear. So the whole thing is pretty clear: if people are thinking about 500+ HP upgrades, the X50 is thrown out money. I talked to a lot of Tuners about that and made lots of price comparisons.