Despite the latest MOTU Fastlane’s driver version started as Snow Leopard compatible, it doesn’t send Midi data over the out port (only manages incoming messages from Midi IN). This problem has been confirmed here and there, however MOTU didn’t release an update since early January.
So my initial choice of the Apogee Duet (419 euros, audio only) had to be changed to an audio and Midi interface instead, the Echo FireStudio 2 (184 euros). No latency problem has been encountered, a 128 samples buffer works nice, that is (as reported by Logic) 11,4 ms round trip (recording and playback through the mix), in fact 5-6ms latency when using virtual instruments.
The sound seems more powerful (better dynamic) and more present for bass than with Echo MIA PCI card (that could achieve 256 samples on the G4), despite the later perhaps sounding clearer and more opened/precise (less mudded, less masking) – to be confirmed however. There is to note that the mac mini’s integrated converter sounds very close, and perhaps more pleasant.
Finally the Firestudio doesn’t require any driver.