Starting OG12 today

For me, the ideal comibination is OG 12, OG Verbal, and OG Quant. Along with the two GMATPrep tests, that's enough official material. If you need more challenging math practice, look into GMATClub Tests and Jeff Sackmann's math sets. For verbal, LSAT questions are an option (especially for RC).

OG10 has some advantages, such as the amount of questions. However, later versions have more difficult problems. There is also chance of seeing OG10 questions on the GMATPrep, from what I've read.

I practiced verbal and quant questions on the same day. When I just focused on one topic, I would burn out more quickly. Switching between topics allowed me to keep going and stay mentally fresh for a longer period of time.