In general, an aluminum bike will be lighter than a steel bike, and that's typically the reason people will opt for them over steel, although both steel and aluminum bikes can often be found at the same price-point. Also, steel typically affords the rider a more comfortable ride as aluminum is known to transfer a lot more "road feel" (read vibrations) to the rider. The seller describes the "alloy" version as being only a pound lighter than the steel version, not enough of a difference to justify the added expense in my book. Can't speak to the quality though, never come across this particular brand. I'd scour the net for reviews before purchasing and try to see if anyone mentions any specific differences between the two bikes.