You have decent choices on lots of equipment in ATL. I would go to the crew resources section on Deltanet and download the projected category list for next May. When you look at narrow body FO's check the number of CA's in the category to get a feel for how many new hires will be added between now and then. For pure quality of life MD88 is the best choice as it looks like you will have well over 500 guys junior to you. A320 FO could also be a choice for more money.
I also would not discount MD88 CA. Looks like enough guys below you to control your days off. You can drop a considerable number of days and still make what a FO would. Bid weekends off on reserve and snag a GS once a month and you are way ahead.