difference between 750GB Serial ATA Drive @ 5400 RPM and 500GB Serial ATA D

The Solid State Drive has much faster Read/Write times than a regular drive and has no moving parts, the trade off is they are relatively expensive for the volume of data stored. http://en.wikipedia.org/wiki/Solid-state_drive

The 750GB 5400rpm drive holds more than a 500GB 7200rpm drive but spins slower so the read/write times aren't as good. They both use about the same amount of power and generate the same levels of heat.

It all depends on your type of use as to which is best for you, a lot of folks find a 500GB 7200rpm drive very adequate for general use ...

In my opinion the 7200 RPM option is a good common middle ground if storage space or budget are of concern. Though to be honest, my new MBP came with a 5400 RPM drive and for basic use I don't really have anything to complain about but I'll likely be swapping it to a 7200 RPM drive soon.

7200RPM is often what people in audio production will go with, and thats a world where not only large storage capacity is needed, but also good through put and decent read/write speeds are a requirement so that there is no latency when monitoring the recordings.

The more I read about SSD, the more I keep coming back to this idea that the technology still has some developing to do, and as it gets better and storage capacities increase, the prices will likely drop too. There also seems to be some serious compatibility issues with OS X on the new MBP's (check these forums). Though to be fair there are some success stories as well.