berries n cherries:

Your tree won't get 20ft tall in a barrel. It might get 8-10ft. The barrel will keep it that small regardless of the rootstock. G5 might be better if you can find that combination. But really the only advantages I can see would be that the roots might not get root bound as fast and G5 is more tolerant of wet soil.

I've grown many fruits on rootstocks like Lovell and Nemaguard in pots. They've worked fine even though they are vigorous and sensitive to wet soil just like Mazzard. If your soil in the pot drains well Mazzard should be OK.

Thank you so much fruitnut. I'll keep the tree.