Ok,
I tried the GT-8, the XTlive and the GNX 3000.
The XT sounds good, but variation seems limited, unless its just varying overdrive sounds you want. Also the switching isn't seamless.
The Digitech is great fun, especially the whammy, but the sounds are too fizzy/thin for my taste.
The Boss can be endlessly tweaked by the user and the effects are by far the most varied. Most importantly; to my ear it has a much higher sound quality. Considering I love the sound of my Marshall , and I just want a good variety of effects for solos in our more unusual songs- it has to be the Boss unit for me.
This research and trial has reminded me how much I love my Marshalls.