I fell into the dark side of PCPs for which there is really no return (though I do sometimes like the challenge that spring guns afford) . As far as quietness and shootability, nothing (except for older 10 meter target rifles) come close to the PCPs. The newer PCP 10 meter target rifles are quite loud compared to the old spring and single stroke pneumatics. But, these rifles are a bit underpowered (for squirrels) and louder than things like the Benjamin Marauder.
The newer stuff that has recently come out in the PCP world looks quite promising for those on a budget and with lower precision expectations. With the advances in technology, many folks are starting to expect sub MOA groups with their airguns. However, that takes a lot of doing to get there. For getting a tree rat at 30 yards reliably only requires about 3 MOA, a modest rifle will do that with ease.
In the modest category (in terms of price), break barrel spring guns seem to rule the roost. But, these seem to be marketed with speed and power in mind which is what you don't want. You want modest speed/power in these and you want them fairly heavy. Where again, you wind up in the premium spring gun class---which are now in the same ballpark as the lower priced suppressed PCP airguns.
The chief disadvantage to the PCPs is that you have to have some way of filling them. This means either investing in SCBA or SCUBA tanks, or a hand compressor---an additional initial cost.