iPhone 4/5.0.1/current Redsn0w is a pretty stable setup. I can easily see staying put on this until it's time to upgrade the phone--I sat on 4.2.1 for months after the 4.3.3 JB was released, so I'm not one to update every time.
I can't speak to the 4S/Absinthe experience, but I wouldn't hesitate to recommend redsn0w/pwnage for the 4. As others have said, who knows when a 5.1 untethered JB will be available?