I'm sure the Yamaha is a great box. Yamaha doesn't make anything that's bad. Really, they all are. I was only showing my own biased tendencies. If I was switching from the VF160, I'd be tempted to wait for a Fostex 24bit version, or get one of the Korg XD units. To me, they look great.
One thing to try: Give Yamaha tech support a call and see what that experience is like... Korg... I say this because we know Matt at Fostex is nice, but he's only one guy. I know Korg has had excellent tech support (usually a live person right away). TASCAM is apparently not good at all.
Lastly, will wav files saved on your Fostex be importable to a new machine? I'd assume they should be, but it'd be nice to know for sure.