About a year ago, I went through an amp-buying phase, and at one time had five heads to compare! The HD350 and the Shuttle 9.2 were very similar to each other in that they are both very clean, flexible and high quality.
Personally, I found the Shuttle to be a bit more intuitive in that you could predict what each turn of the knob will do, so it's easy to quickly dial
In a tone. The HD350 took a bit more fiddling to get where I wanted, but I suppose it wouldn't take long to get familiar with its workings with some effort.
In terms of tone, like I said, very similar. With either amp you can get virtually any clean tone you'll ever need. If I had to choose, I'd probably go for the HD350 for that extra solidity in the low end.
Volume? I found the 9.2 to be louder than I'd ever need. and the HD350 to be just loud enough.
Another handy bonus for the HD350 is the Notch control, good for eliminating boom in a room if that's ever an issue.
If I had to choose, is go for the HD350 again, but there's not much in it