Need advice. Nikon or Canon mount?

[Sami Sinervä]

I would take Canon 24-105mm which is solid replacement for RED 18-85mm and de noise in post if sometimes the lens it too slow because of its f4 minimum stop.

Agree on that. A great lens I've used a lot that looks really nice with IS support, stabilizing hand-held shots slightly.

Be aware that if you shoot full 5K, Canon EF-S glas can cause vinjetting.

[Pawel Achtel]

Or, get a Canon Mount and a Nikon adapter.

[Nick Morrison]

Agree on that. A great lens I've used a lot that looks really nice with IS support, stabilizing hand-held shots slightly.

Be aware that if you shoot full 5K, Canon EF-S glas can cause vinjetting.

You might consider the Contax Zeiss 28-85 zoom too, its a 3.3/F4 lens. All manual focus (with huge focus throw), and unbelievably sharp....rivals some primes.

And won't vignette at 5K. It's full frame.