Well, the progressive progressively adds alky based on some varying (hopefully indicative of load) source such as boost, MAF, etc... Non progressive means the alky is injected in the same quantity across the entire boost range. Non progressive results in less power when you have a varying boost i.e. more pronounced with street driving because you end up injecting too much at certain points. At the track, boost is generally much more constant from launch till end.
Some progressive kits, since they are controlled with 'smarter' electronics, are much easier to tune instead of manually adjusting a hobbs under the hood. More money = more adjustability = more control = more power
Phil