I have 2 spare hdd one is WD7502AAEX and the other is ST92505610AS and was wondering which hdd to use for os? WD drive is SATA3 but ST drive is SATA2, my motherboard is ASUS P5QL-EM that only supports SATA2 so both drives will be running on SATA2... looking at passmark tests WD drive scores a lot higher than ST drive but as i imagine WD drive is running on sata3 at that point when tested so the question is - if both drives ar connected to sata2 which drive is faster? i would guess that ST drive cuz its hybrid drive but does that mean if connecting both drives to sata3 mobo will WD will be faster than ST hybrid drive?
The sustained transfer rate of a hard drive is determined by how much data can be crammed onto a track and by the spin rate of the drive, and no 7200RPM drive can get anywhere near the potential throughput of a 3Gbit/sec SATA connection, let alone a 6Gbit/sec one.
Choose the drive with the highest native performance and don't worry about what kind of SATA connection it has. The Momentus drive will be able to cache some of the data in flash memory and so will likely have faster average access times overall even if it does have a slower transfer rate. If you're using the drive for something that benefits from fast access times (such as the OS disk or a disk to store application programs on) then it's quite possible it will perform better than the other drive even if it has a slower maximum transfer rate.
The Momentus XT is a better OS drive than WD Black. The SSD cache on the Momentus will give you faster access and response times which is the main reason to use SSD over a HDD. The WD Black is probably beating it on transfer rate but that hardly matters. Momentus XT performs around the level of a VelociRaptor so it will beat a WD Black for OS.