So, I'm taking jazz band this year or next year. (Probably this year)
I have two trombones: a school one that is amazing, and it has an F attachment. The other is just a plain peashooter. I've been told that playing on a peashooter is better for jazz because of it's solitary sound. But, I'm very used to playing on one with an F attachment, since I've played that one for so long (and it's got a HUGE bore.)
So, peashooter or F attachment (for jazz, remember.)