If given a choice - 737.

As a new hire you will have a much higher quality of life - faster - on the narrow body fleets. 756 is a great airplane (intercom excluded) and the flying is great but there is a reason we force new hires into that seat on a cyclical basis. Seems like we force tons of new hires on it and exactly two years later they all bid off so they can have seniority on 737.

Reserves will get put in whatever open seat needs assigned. There are some line holders that bid for the IRO position exclusively but most tend to prefer the flying seat and therefore IRO is typically a junior lineholder. Many junior line holders will be in pickup requests to trade up if the flying seat becomes open but many just don't care - it all pays the same but first break makes for some really long nights on the backside of the clock.

Reserve doesn't get to play in the premium pickup/trip trading. Seniority gets you more time off, better vacation options, the option of bidding reserve in the slower months etc.

This could all change in a month or two depending on what is in the TA and if it gets ratified.

If you live in NJ - don't go to DAL. Newark isn't a great base but by living there you will have tons of opportunities for premium pay as a LINE HOLDER.