read up either on here or on sheldon browns page on bikes and metals... the gist is that it comes down to the geo and tubing size... if you had a large tube steel frame it would be stiffer than AL but much heavier, so they use AL to cut the weight down... there is lots on this topic all over the place and lots of old myths too... im going with AL myself after reading up on it a lot and talking to many people on here...