Mulltron, you're trying to balance the phases, and that's good, but I don't think it's a big concern here. The pros can correct me if I'm wrong.
I'd simply run #8 between the panels, from a 50a Double Pole Breaker at the MAIN to the subpanel. Then, you can wire your subpanel branch circuits as needed.
In order to balance the phases, if that's what your concerned about, understand that as you add branch circuit breakers down one side of the panel, each breaker is automatically placed on the opposite phase (or Hot Bus) as the one before it, so that (in a 10 circuit panel, for example) 1, 5 & 9 are on the same bus, while 2, 6 & 10 are on the other. In a 10 circuit panel, 1, 5, 9, 4, 8 would be on one bus, while 3, 7, 2, 6 & 10 would be on the opposite.
I won't address the the advantages of 240 vs 120 except to say almost everyone provides a subpanel (for general lighting/load) with 120/240 so that they can run both 120 and 240 volt devices.