I would go as wide as you can without running into vignetting or corner softness. If your lens is sharp in the corners wide, then vignetting can be fixed in post anyway. You want to get as much light on the sensor as you can to reduce noise. Noise is not a function of ISO, but of exposure. The less light that is hitting the sensor, the higher your noise will be. ISO actually reduces noise (for example if you took shots with the same shutter and aperture and at iso 400 and 800 and then in post upped the exposure of the first by 1 EV, the iso400 shot would be noisier). That is why long exposures in low light can be noisy even at ISO 100.