Low quality is about what you'd find if you've downloaded a ripped movie off of kazaa. It's acceptable, but painful to watch at sometimes because it is so bad. Most of the low quality episodes are in 320x240 format with varying frames per second. Larger files will have more frames per second. Some only have 15 fps, but that isn't at all indicative of the quality. Some of the 15 fps episodes of the low quality are actually better looking because they have twice the data to work with because they are updating the picture half as often.
High quailty episodes on the other hand are quite good looking. They look great when I watch them on my computer screen and are easily better than VHS quality. The only down side is the large file size. They're about 160 a piece and take four times as long to download.
The question of which is right for you is a personal decision. You should look at factors such as your available bandwidith, how much space you have left, and where you'll be watching the files.
Even though I can download the entire HQ season six in about three hours, I mostly have the low quality seasons because the large file size and because I'm not that picky about what it looks like. As long as I know what's going on I'm fine.
Hope this helped,