If you're going for speed, Seagate Momentus XT 500GB (7200RPM). It caches most-used sectors on a 4GB build-in SSD, so the sectors stay on the SSD rather then get cleared when the system gets powered down. Bootup- and Application load seem to speed up significantly (In Seagates Test-Video it's faster then a Raptor X 10000RPM-Drive, nearly as fast as a real SSD, but... ya' know). Performance increases over time, as it finds out what data to cache (Bootup-Files, in your case Safari, iPhoto, Movie Player etc. if you use that a lot). You get nearly the speed of an SSD at nearly the price of a HDD, and the HDD is a big factor in overall system speed. (Things like putting the virtual memory onto a Class 10 SD(H|X)C would go beyond the scope in this topic)
If you go for storage, WD Scorpio Blue 750GB Advanced Format (There are 2 versions out there, 5200 and 5400 RPM. Be sure to get the 5400, it's just faster). It's even slower then a virgin Momentus XT (it can compete, though), because of the RPM, but if you got lots of files, that is the disk to go. The Advanced Format means that sectors are 4kB instead of 512b, so you save overhead because you can fetch eight times the data with one command. Speeds things up a little for big files, is not recommended if you have many files smaller then 4KB, because each file would use the whole 4kB. But as we think in Megabytes, Gigabytes and Terabyte (and sometimes Petabyte) these times, that shouldn't cause headaches, the Advanced Format increases the accessible data on the disk to 96% instead of the 87% because there is less meta-data between the sectors. And all this is supported by at least Snow Leopard, as well.
They cost about the same (±20$), so it's just a choice between speed (and a little lower battery runtime because of the faster spinning drive) and capacity.
If your device fits 12.5mm-HDDs, you probably want the 1TB-Version of the WD, if you choose for capacity.
SATA II is just "nice-to-have", as HDDs rarely saturate SATA and thus not needed, unless you go SSD (Even for the Momentus SATA I is sufficient), but all drives are SATA-II.
The Scorpio Black 500GB (7200RPM) would be the low-cost variant for speed, as it's faster then the Scorpio Blue but has no cool SSD-Cache, and costs about 3/4 of the Momentus XT.
Just IMHO.
Regards