Quality of the exhaust i suppose.
You buy cheap, you get cheap! Companies such as powerflow etc have been around for years and have had a lot of bad press, mainly coming from cars like novas/saxos etc. All young chavvy boyracers tend to have a powerflow exhaust hanging from their souped up 1.2 novas.
Supersprint has a lifetime guarantee and is designed and developed in Italy, it is imported into the UK hence why its the dearest out the bunch. Im sure miltek also has a lifetime guarantee and so many people have got the miltek system for it to live upto its good reputation.
Blueflame has also been slated to death on here, they made the first batch of A+M i believe, and there was uproar on this forum about them! They released a 2nd batch under there own brand name at a cheap price and again there has been problems!
Better stick to brands you know and that have a good reputation, its the same for remaps, you should only really touch Revo/Jabba/APR/AMD and CC. There are plenty of others about like e-maps that are half the price but they have not been tried and tested 50000+ miles, all the companies above have!
At the end of the day the old chiche is correct, buy cheap, get cheap, buy twice!