I've actually never had success recording with Sound Studio - I'd get consistent five-second "blat" noises between 30 and 40 minutes into each recording I did. At first I thought it might be my deck(s), but ruled them out. Then I thought it might be the OS X driver for my U2A (which was in beta at the time), so I waited for the non-beta version, and still had the same behavior, so I resigned myself to using OS 9 for audio until OS X behaved itself, figuring it was the driver and not my software.
Just last week I tried Spark after seeing another taper use it live, and it recorded without a hitch...but I couldn't figure out how to edit with it, so I just used SoundEdit 16. But I'd much rather use one program for capture and edit...and Spark is so much faster than SoundEdit or Sound Studio, which is attractive to me - when I used Sound Studio in OS 9, it'd take a minute or two to write a temp file when I did a cut-paste from one file into the next, and SoundEdit 16 has to redraw the wave form every time you scroll it, which is really irritating. I'm interested to see what's new in Sound Studio 2.1, but it's been "coming soon" for months now...
--Dave
ps. For what it's worth, there's still a bug in the OS X driver for the U2A - it won't light up the input light unless you click on "Analog" and then "Digital" again in the control panel. I've emailed them about it twice and didn't get any response...