I actually used to play some light gun game on the wii and it was reasonably accurate. It would be more or less the same accuracy as the wiimote since it works on the same tech.
Given that they work on IR you need to be careful where you are using it, as any other IR sources, like direct or even indirect sunlight, will mess with the accuracy, but if used in the right conditions the wiimote or other any other IR light gun works perfectly fine.
You also need to remain in the same position as where you calibrate the gun, or re-cal if you change position. If you move around the gun will lose accuracy. Also don't stand too far back.
I guess the other option is the PS move. I've never used one before so I don't know how accurate it is.