FYI, the Air Tight and MSL cartridges are both designed by Matsudeira (if that is the correct spelling), as was the aforementioned Audiocraft AC-03 and even earlier models like the Entré EC-30. Based on my own personal experience with the last two and various reports about the first two, there appears to be a clear family resemblance.
By improving on his design and using new materials, Matsudeira has managed to bring down internal impedance and increase output. So at least in theory the latest Air Tight and MSL design are superior. I don't know to what extend this translates to better sonics, but again I've read reports that the MSL's don't sound very different from the previous Audiocraft AC-03. If that is correct, these are bargains on the 'previously cherished' market. I found an 'as new' sample for around $1000 and at that price you won't be able to do much better.
By improving on his design and using new materials, Matsudeira has managed to bring down internal impedance and increase output. So at least in theory the latest Air Tight and MSL design are superior. I don't know to what extend this translates to better sonics, but again I've read reports that the MSL's don't sound very different from the previous Audiocraft AC-03. If that is correct, these are bargains on the 'previously cherished' market. I found an 'as new' sample for around $1000 and at that price you won't be able to do much better.