loose footed or track mainsail
You have way more abaility to adujust a loose footed main. Depending on the configuration you may find tht your outhaul does next to nothing on a slide on main. I''ve seen this on some sails - haul the outhaul as much as you want, there is just too much friction to really pull the foot tight.
With a loose foot the outhaul can have a huge impact on your sail trim. This will have more of an impact downwind, since you can really loosen the sail up a lot.
Having had both, my personal preference is for loose footed.