I have only shot them at the store, but the nitro piston rifle does not have the spring noise, or the piston noise. (I know it has a piston, but you can't hear it, or the "wragnnnnggg" noise when you fire a standard break barrel air rifle.)
I have had the pump type air rifles for decades, and they work well, plus you can adjust how fast you want the pellet or bb with how many pumps you do.
I'd say the pump rifles, and nitro were about the same on noise, but you don't have the "pumping" noise to even deal with.
If I buy another air rifle, I'll get a .22 nitro piston type with a light composite stock.