All things being equal... I would prefer to have a flash.
It's a feature, plain and simple.
If you take a picture with the flash and feel it's washed out, take another one with the night mode instead then pick the picture you like best. Rarely is it impossible to re-take a picture... Just tell everyone to keep smiling while you snap a second pic.
If you need to see something in the dark and have a flash on your camera to use as a flashlight... how can this be seen as anything but a handy feature? Having a flash doesn't make the phone any bulkier and doesn't cause any reliability issues.
Flash = Good, even if it's useful only once in awhile.
No flash = No choice at all, which is sucky.
I def feel that Samsung should have included a flash on this phone, it's a shame really. Either way, I will be using the Galaxy S 4g until the S2 comes out.