You have to be careful that one compares apples to apples here. The 790 is lighter, but not smaller dimensionally. I'm not positive, but I'd bet the 815 is 24" back. The CK27 24" back is something like 1900. The CK27 lists total hydraulic pump gpm. Most manufacturers separate power steering from auxiliary hydraulics. Again, not sure, but Id bet the JD number is just auxiliary. The CK auxiliary is something like 7.2 gpm.
Still the CK clearly out specs the JD. The JD is an older, tried and true design that is a basic dependable tractor. Doesn't have the bells and whistles that the newer tractors have.