Hi Chris, being totally blunt I would recommend you forget about the Marshall. Th reason being that in all my fifty odd years f being a professional guitarist I have yet to plug into one that pleases me and I have used a few.
The Katana however is a different beast altogether. I used the Mk1 head for two years with a Vox 2 x 12 cabinet an it was terrific, then I went super lightweight and bought the Fender Tone Master that you can carry with your little finger. I missed the Katana though and recently bought the Mk2 100 head that has a small speaker for home use as well as amazing effects. I can get all the Shadows echoes I need from my Katana as there is the option of using four echo heads to get tall the patterns. Also there is all the tone shaping that you will ever need on board when you connect the unit up to your computer with a USB cable into Boss Studio.
Should you wish to use your Zoom G3 instead that's no problem. You just set up a Vox sound and plug the Zoom in as normal and away you go.
I very rarely go backwards with my gear, but I knew the Katana Head very well and got the most from it, the new one is twice as good so I had to have another.