Here's my two cents on this...
Like cdroge stated, it really depends on where you live and who is teaching. However, I feel Mambo is definitely more defined and consistant than salsa. Salsa is more of a club dance, and can vary greatly from place to place. A lot of salsaleros will tell you that salsa is more grounded in the hips and knees than mambo which is seen primarily as a ballroom dance. There's more of an open or non-existent frame with salsa, while the mambo frame is a typical latin frame.
As far as steps, almost everything is interchangeable between the two. Salsa does start on the one beat, while Mambo is on the two (just like the check forward in cha cha and int. rumba). Therefore, you hold on the 4 in salsa, and on the one in mambo.
Salsa music might be a little faster, but not by much. I think it's also harder to here the beat in a lot of salsa music. That might be due to the incredibly loud volume the music is usually played at in clubs. I've danced a lot of mambo in studios and some salsa in really authentic Latin clubs, and I personally find Mambo easier to dance. I think the fact that you start on the two helps a lot to slow the dance down. You can wait for the one, and then begin to move on the two beat instead of trying to anticipate the one beat and move on it.
Mambo is also one of the five American style ryhthm dances done in ballroom competitions while salsa is usually reserverd for clubs. (Yes, I know, there are salsa competitions as well)
I'm sure many others will have opinions that agree and disagree with mine, but like I said, it's just my two cents. My favorite response on this subject that I've had from professional instructors is someone who told me that salsa is what you dip your chips in.