The normal one is generic meaning that it was put together in a 3rd world country, has no name, and will not last long, give the power output it says, as for all you know they could of tested it in a feezing cold country, at the coldest time of year, and then tested its outputs, and wrote down the maximum it will give out. The Amps on the 12v are dangerously low - meaning very poor stability.
Where as the more expensive ones, from good companies have been tested in a warmer temperatures, like the ones you would be in, and measured the output, stating the minimum average and maximum output. They have a repectable/very good amps on the 12v. They will not blow out on you, where as generic are a ticking bomb, waiting to blind you with a flash, while pouring out smoke, and possibile taking out the whole PC.
It is extremly worth spending more on the PSU. If you can afford it, get a OCZ, Antec, higher end Enermax. At least get a thermaltake, as many users here use them, and are extremly reliable.
Cisco CCNA, Comptia A+, 1/3 through CCNP