I own the P89 and have held and felt the trigger of a Sigma. I think the Ruger is a better made gun than the 9V and is better built. The P89 has a longer track record to, FWIW.
If you go with the Ruger, try to find an all metal (other than the grips). I recently found they made them with a polymer frame, but all I have actually seen "in person" have been aluminum.
As for accuracy at 25 yards, they should both be able to kill a wattermellon without missing more than a couple of times. The P89 will!!