I think the torsion box will be flatter, and more stable, if built properly. Don't use 2x4's for the frame, though. Just use 3/4" plywood. It'll be more stable.
I'd hold off building the cabinets if possible, until you know how much room you will need for your control box, which can take up a lot more space than you think. I'd also plan on putting the PC in a sealed cabinet, if not the control box itself, as dust is always an issue.