For some reasons in test, the dometops retain better velocity,accuracy, and killing power down range. The .20 caliber/5mm is the best balance of velocity and killing power according to Beeman too.
I had a 5mm Sheridan as a kid and killed the heck out of stuff. I have a 22cal now and at sixty yards it really has killing power over the 177.
This is all science and testing, I didn't make it up.