What do you use your Macbook for?
If you use it for gaming, and Web related tasks, Ok the XT will have few speed advantage to choose.
But if your mac usage is for Heavy Work, or something you depend on, then go for Scorpio ( I've a Black 750, but the newer 1TB Blue, is better, saves power and also have same effective speed).
MOMENTUS XT= Faster at boot (and few app launch), but not as reliable as a STD Spinner, too many complaints about long term wear, data integrity and system stability, 4GB cache is not enough for advanced applications (photoshop, Autodesk, etc), but fair for Web usage.
Scorpio Black 750GB@7200rpm = faster after booting than XT (YES IT'S TRUE...) but Drains more power and Heat as Hell on heavy usage (don't worry your mac will not melt).
WD Scorpio Blue 1TB (5400rpm) at the moment is the best Replacement for original MBP HDD, despite it's lower RPM, coz has the double density, each Spin equals to 2 spins of the Original 500GB, so its like having a 10k RPM HDD on your mac, and drain less power than original HDD, but the Scorpio Black 750@7200rpm still the fastest 2.5"HDD (combine density and spin, plus twice cache).
Momentus XT performance gain dies after booting, it's great for some circumstances but depends on the kind of software you use ( I use Autodesk Autocad, Xcode and Photoshop very often, the momentus XT 4GB cache just cant handle this, I would need a 16GB cache at least).
Anyway, the best performance and battery saving improvement you can do is to replace the RAM with 2x4GB modules (8GB), this is the real Speeddy Gonzalez inside your mac.
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Ahh, also consider that MORE space availability is another long-term time saving.
When your HDD is getting full, data fragmentation becomes the main speed trap, so having more free space will delay the need for defragmentation process (saving you hours).
Also you'll not need to replace the HDD as soon coz you need more space.