They are both fine pistols.
I have one of the early model RAMI's that had to go back to the factory because of feed and eject problems. Supposedly, CZ has rectified those problems in later models. It is the most accurate 3" pistol, and the easiest to shoot well, of any I have tried, although I have not had my hands on the P-239. I can very easily put an entire 14 round magazine onto a paper plate at 25 yards with my RAMI, and I cannot do that with all of my 4" guns.
On the other hand, I have no personal knowledge of anyone having to send any Sig back to the factory, and they are also known for their accuracy.
EDIT: About the conversion kit - I don't know if the CZ Kadet kit is supposed to work on the RAMI. I stuck the one I have for my CZ-75B on it, for grins, and it seemed to fit fairly well, but of course the magazine sticks out of the bottom pretty far, but I'm afraid to try to shoot it without knowing for sure if it's OK.