MacBook Pro Hard Drives - which one ?
Serial ATA vs Soild State
This is not a "vs." Most modern desktops and laptops use Serial ATA as the interface for attaching internal hard drives, solid state drives, and optical drives.
obviously (by the price) Solid State is much better, but how and why?
If you jolt a HD, there is a small chance that the heads will fly into the surface of the disk, resulting in a bad sector (data loss) or a head crash (much data loss). On HD-based iPods, jolts could also cause harmless but annoying 'skip's (which is why many HD-based iPods read ahead a few seconds, for anti-skip protection). With a SSD, unless the thing comes loose from its SATA and power connectors, or the computer flies apart in pieces after you drop it 5 feet onto concrete, you don't have to worry about this. This makes SSDs a good match (in this regard) for laptops.
SSDs can have very high read speeds compared to HDs. SSD read speed is not bothered very much by highly fragmented (in the traditional sense) files, because jumping from one spot in the SSD to another does not require waiting for a motor to move a read head.
SSDs can have high write speeds, but they also have a performance issue that does not exist on HDs. Overwriting a (say) 512-byte sector on a HD is easy. You wait for the heads to get to the right part of the disk, write the data, and you're done. On a SSD, if you are overwriting a 512-byte area that you have used before, you might have to read-format-write a 16384-byte sector. (All numbers here are educated guesses; for illustration.) That can be very expensive.
is it really worth considering? especially when i'm breaking the bank to get a new laptop anyhow...
If you're "breaking the bank" to get a new laptop, I'd suggest sticking with a regular HD. If you're torn between 750GB, 5400rpm and 500 GB, 7200rpm drives, consider that there are third-party vendors who sell 750GB, 7200rpm drives. You could buy the MBP with the standard HD and upgrade it yourself to one of these.