Do not concern yourself with damage for 'solo play'. There is nothing solo that cant be obliterated by any tanks dps andthey are all pretty close anyway.

Similarly sustain is a non issue for solo play. All tanks have a number of ways to slowly regen their health. All solo content is designed to he conpleted as dps classes and their damage is tuned to that. If it is fit for a dps it tickles a tank. Your passive regen is usually enough to stay alive forever on any solo content.

Tldr: solo content is utterly faceroll for tanks. Just play the one that seems more fun. Gun is busy. Drk is weighty. The aesthetics are pretty clear. If youre not sure try them both then decide.