What's better for low light and indoor wedding videography?

Sony NEX-FS100UK (with 18-200mm SEL Lens)
- or -
Sony HXR-NX5U

What if I can get a shorter, wider-aperture Nikon Lens (with Nikon Lens Adapter, of course) for the FS100?

You will get better responses from the Camcorder guys next door
& I'm taking your question over into that category

The FS100 in theory should be better but it depends on the type of lens you are using. It sounds like what you have is a 3.5-5.6 aperture lens. If you are at the 5.6 part of the lens you will struggle in low light. I do not know what kind of lens is in the NX5U or how well it can process noise which is another factor.

The short answer though is that if you get a larger aperture or constant aperture lens of at least F 4.0 the FS100 should win the low light battle hands down.