cobaltblue:I don't know about the Aqualung but I know for a fact that ScubaPro stands behind their products and are a great company to deal with. I was using a MK25/S600 combo and loved but recently switched to the MK17 with the S600 mainly due to the cold water in the Great Lakes. I can't tell any difference in the way they breath and the fact that the MK17 is sealed and about $100 less, I think that would be the way to go.
how do you like the mk17? i know that it has two diaphragms and is an evolution of the mk16?