I preface this by saying I have no personal experience of any of the machines you're talking about, but...
If the Boss (aka Roland) machine isn't doing it for you, I don't know that you'd consider the Zoom as an upgrade. Roland and Fostex have been in the portable recorder business for a long time and I'd tend to trust them rather than Zoom - only Zoom thing I ever owned was a multi-effect which was so noisy as to be unusable - it was thrown in with a guitar purchase and I later gave it away - things may have improved, however.
Why are you going from 8 channel to 8 channel? Perhaps one of the upper model Rolands / Yamahas / Fostex is more where you should be headed, depending upon what you want it for and financing, of course.
Then there's the PC-DAW route as well, which others will comment on - I've always liked and used standalone devices so I won't go down that path with you.
Cheers