I have been using Mig for 5 years and last year I switched to Tig. I wanted to share images here but this forum system seems complexI would suggest that welding a roll bar for actual roll-over protection is not a job for a beginner. The work should be performed by an experienced and skilled welder, and I would want one who is AWS certified. The tube fit-up, the weld penetration, and the weld bead reinforcement profile all have to be correct. Failure of your welds could have serious or deadly consequences.
Many of the 20+ TIG Welders at the shop I work for have their own TIG machine at home. We only have one MIG machine and it is used for building work tables and fixtures.
Your welder should have settings and capability that exceed your roll bar material thickness.