For ratting Terrier I might be inclined to go for the 3-9x40. Mine focusses down to 10 yards so no problem for rodents!
Regarding the pellet trajectory, due to how low the scope sits in comparison to the barrel, and the ability of the scope to retain a clear sight picture from 10 to nearly 40 yards (slightly further outside,)the ability to reduce the amount of holdover/under makes the gun appear to shoot flatter, certainly more than my Steyr/Bushnell HFT rig with Mosquitos. This obviously will allow me to feel more confident in hitting reducer targets, as the sight picture is much more focussed and the low sitting scope will make the most of the improved POI variance. This usually means having to increase holdover at 40 plus, but for some reason the combo appears to shoot flat out there as well.
I am not saying that the characteristics of this scope defy the laws of gravity working on a .177 air pellet, all I am trying to say is that with a low mount and barrel arrangement in combination with this particular scope, the combination appears ideal for both HFT and hunting at legal limit airgun ranges. All will be revealed on Sunday when I will discover if the scope has had a noticeable impact, as I am confident it will with 15mm targets at up to 25 yards. I managed to achieve joint top score at Oldham last time with a 45ex50 using the Steyr and look forward to bettering that using my preffered Rapid/cheap scope combination.

Andy