You want the Disc for food plots and over unbroken ground penetration is key.
Seven and one half inch spacing gives more float/less penetration. You may have to wait for days with optimal soil moisture to cut. Fortunately, most food plot seeds are strong germinators.
Easy gang angle adjustment via screws is more valuable with 9" pan spacing than 7-1/2" pan spacing, where you have less penetration ability. However, easy gang angle adjustment is worth paying additional to have.
20/20/7' is a good size for your L3540. You will be pulling it in HST/MED with a good bit of throttle, as I do my Tufline. (My L3560 has 70" tire width with R4 industrials.)
$1,200 is a good price. My 20" Tufline was double $1,200.
I have no personal experience with Leinbach implements but from reading TBN they enjoyed a good reputation.
Ring the pans if you bring the Leinbach home, to be sure axle nuts are TIGHT.