Trapper,
I was in the same boat as you. I wanted to move up in power to .22 thinking that .177 wasn't enough. I was wrong.
The .177/.22 debate will go on long after we are gone.
The .177@850 FPS = about 14 FPE (pounds of muzzle energy).
The .22@720 FPS is around 16 FPE
For tree rats you need about 5 FPE. With the .177 your good to 50-60 yds. with .22 even further (depending on which calculator you use).
If your shooting them at that distance, you don't have a pest problem, you have an addiction

Most of my in town shooting is 25-30 yds with my B-26 and with those numbers even head shoots go clean through.
In the country, we all shoot, so I still went .22 just for something different. Being frugal

... I went B-25.
The B-25, scope, mounts and tuned from Mike M is cheap for what you get.
If you tune it yourself, even cheaper. You saw the link a few posts up right?
It's the clone of the Diana 34 which is a known power house.
We all want good, power & cheap, but once you get going you want to shoot more and more and more.
Just from a frugal point of view you will spend a lot less on .177 pellets than .22's, and If your on a buget that's the way I'd suggest.
There are a lot of good Gamo's out there, but they're more money to start with and still need a $ trigger job, scope, mounts.
Just I thought...are there any local airgun dealers by you that might let you try one out back or something?
All this talk about which one to get could be solved with a little hands on. It might just click after trying one.
Good luck it's never an easy decision!