In SFO the 320/737 will offer fastest upgrade to line (caveat below). For a new hire the 320 will be the easiest to learn and fly. If you’re a fighter guy or have flyby wire experience it’s an easy transition. 737 has the best flying, airplane is much more manual and work load intensive.

As a new hire I’d avoid the 756 unless you’re previously typed on the airplane. All my buds tell me it’s hands down the hardest school at TK. Also you’ll be seat locked from going down for two years (up anytime). The flying is predominantly two day domestics which for a commuter can be a pain, and they’re to EWR and BOS. The fleet is way undermanned in both SFO and EWR so line holder status could happen quick. EWR 756 has the best flying at United. Last vacancy bid both coasts were about 20% unfilled vacancies on 756.