You can bid in PBS for what you like, but seniority determines what you will get. You will get senior faster on the 737 rather than the Airbus.
Reserve schedules vary widely. You can expect 2-6 days on reserve, with probably 2-4 days off after that. Minimum days off on reserve is 12/13 (at least I think it's 13 in a 31 day month). The reserve trips are a mix of regular trips, broken trips they've put together, and short calls/field stanby's.
Junior trips are mostly destinations people don't want, redeye's, or trips with lots of legs. The trips range from 1-4 days, generally.