Some things come to mind:
Tri-borane is in a low valence state so easier to ionize.
Borane is probably cheaper and more stable.
If no isotope separation occurs for B11 before combination w/H,
Extra B10 in tri-borane is wasted. Interestingly, borane is made by laser ablation of boron in hydrogen. The added step of laser isotope separation is obvious.
Equal portions of boron and hydrogen assumes 100 % conversion. If excess ions are left, hydrogen is preferred, since it increases chance of boron collision.
