Compressing a Slow Drive for Performance – Is It Effective?

A generalisation isn't useful, but mostly when talking similar models/same series I'd say the larger drive would be faster due to higher data density in some way (be it more platters and heads, or just denser platters).

The bigger model would likely be newer as well and could benefit from firmware and other production improvements.

This is even more true in the world of SSD where the larger capacity options are generally faster due to more parallelism. On the other hand, sustained throughput isn't always the important factor of an SSD compared to mechanical drives but rather the low-latency on small random access read/write - which will be the same in most scenarios regardless of the number of chips.

I have never found raid arrays really make that much of a positive performance boost for every day use. They certainly don't improve access times and that is what you tend to notice most in general use. I would also avoid raid-0 unless you don't care about the data being stored.

An SSD is the way to go to get a noticeable speed boost. Use a modest sized (60-128GB) one for a boot drive and a larger mechanical drive for bulk storage. If you want your games to load faster they should be kept on the SSD in which case you might want to get a larger drive (120GB+).

Almost any fairly recent SSD should be OK but Intel is best followed by drives based on Samsung or Indilinx controllers (Samsung are probably safer and Indilinx slightly faster). Cheaper drives based on JMicron controllers are OK only if you are on a very tight budget. If the price is cheap and there is no mention of the controller then assume it is JMicron based regardless of how impressive the specs might seem.