.25 Marauder will indeed whack em dead. But in an "urban" environment, you really are shooting the equivalent of a .22 CB round. The aerodynamics aren't as good, but still, maybe a bit much unless your backstop is literally bulletproof.
As Boxer/BigStick said, there is nothing like a FWB 300S. But you're trading power for pinpoint accuracy. I've heart shot squirrels with the .25 Marauder and they just fall out of the tree. Not as likely to happen with the target .177. Figure on head shots only. I've seen them from $3-400 right now, depending on condition.
I've got a .177 Marauder for avian pests, and it makes the .25 Marauder sound positively loud in comparison. And the .25 is anything but truly loud. Accuracy not quite like the 300S, but still quite good, and with heavier pellets, about double the energy.
But the .22 Marauder "pistol" might very well be the best of the lot. About the right amount of power, and in a nice compact package. Do a little hand pumping and get some exercise before you shoot. This is purely anecdotal on my part though. While I own the first three pellet rifles, I do not own the Marauder pistol.
