The real difference between two lay in their very distinct manufacturing processes. Aluminized steel involves taking some sort of standard-grade steel and using a galvanizing process to coat the unfinished steel in a layer of tough, corrosion-resistant aluminum alloy.
Stainless steel on the other hand, are purpose-built steel grades that have been alloyed with chromium, nickel and/or manganese to create a completely corrosion-resistant material from the ground up. Several different grades of stainless steel exist, with either 304 or 409 stainless steel being the most common for exhaust systems. 304 steel is part of the austenitic class of stainless grades and are identified by having at least a 16 percent chromium content with trace amounts of nickel and manganese and have extremely low carbon counts to reduce or eliminate corrosion issues at weld points. 409 stainless steel is a ferritic class steel that contains more carbon, but less chromium and almost no nickel that makes 409 systems more susceptible to corrosion; but stronger, more heat resistant and are easier to machine than other grades. Both 304 and 409 steel are more corrosion-resistant than aluminized products, but it comes at a price tag that may be several hundred dollars more than similar aluminized systems.
Neither type of exhaust material has a performance advantage, and the only consideration to which type of system to use should be the climate in your region and your specific application. Your driving habits are just as important as your weather when it comes to exhausts. For example, a car from New England used as a daily car for all year round can manage to completely rust out an aluminzed exhaust in only 2-3 years. However, customers from a similar climate zone that keep their cars in the garage and only use them at the track can keep an aluminzed exhaust looking flawless for the life of the vehicle. In the end, those who like snow, blizzards, Nor'Easters and extreme humidity should stick with stainless steel. People who bask in the sun on regular basis aluminized exhausts will save money while being perfectly viable for everyday use.