[jmg]

I've shot one wedding as a favor to a friend before and I had some GREAT results with the 24-70mm f/2.8 that I rented. Paired with my Canon 6D, and 580ex II Speedlite and shooting f/2.8 at 1/50 I was able to capture all the action sharply with no motion blur and still have the backgrounds pop. Now I have a high profile formal birthday party that I agreed to do. The problem is I don't want to rent another 24-70mm f/2.8. But I don't want to buy one either, since I don't have a real use for it outside of these parties. Keeping in mind that I want the option to have a big aperture so that the flash doesn't completely kill the background, which of these lenses that I already own should I bring?

Pros/cons of each:

24-105mm f/4L:
Pros: Perfect focal range
Cons: f/4 will make my backgrounds darker than I like since the flash will be doing more heavy lifting, and the venue is a very dark place.

16-35mm f/2.8L
Pros: f/2.8 is perfect for this lighting without having an overly shallow DOF.
Cons: The lens is useless for people (my main subject) at wider than 24mm. So only 24-35mm are useful and not as versatile as the 24-105.

35mm f/1.4L
Pros: Low light performance will be amazing
Cons: A nice all around focal length, but not too flexible

50mm f/1.8
Pros: Not too narrow on a full frame
Cons: Still might be too tight in some situations. The 35mm will outperform in every way, but I thought I would throw this in as an option.