cropped or full frame for one shoot

I'm doing some outdoor photography for a kid's Christmas event tomorrow from 5pm to 9pm. Sun down of course, so there will be some lighting, but I'm not sure what or how much.
Would I be 'safer' to use my Canon 40D/ very good 17-55 f2.8, or the Nikon 750/ 16-35 f4 (also very good lens, but f4, for that evening shot?
I do have speedlights for each camera.

Thanks

Whichever combination focuses more reliably in low light, and handles medium-high ISO better.

f/2.8 gets more light onto the sensor, but f/4 gives you more room for a little focus miss to not ruin the image.

If it is people wandering around outside (IE candid group shots) then I would go with the D750 and the f/4 lens - with a flash on. You don't really want to be shooting groups of people at f/2.8 and the flash should be able to illuminate small groups enough for you to shoot at a narrower aperture (with a raised ISO).

Dan Marchant wrote in post #17805380
If it is people wandering around outside (IE candid group shots) then I would go with the D750 and the f/4 lens - with a flash on. You don't really want to be shooting groups of people at f/2.8 and the flash should be able to illuminate small groups enough for you to shoot at a narrower aperture (with a raised ISO).

This is the direction I would go.

However, I would say that you can go either way. You can always stop down the 17-55 lens (sharpens up a bit too). Add some ISO.

But the bottom line is, take the speedlites.

Basically whatever system you have that works with your speedlites is the one to take. Ultimately this is what trumps choosing which camera in my book.

Very best,

A lot of people were differentiating between the DOF, so it is worth pointing out that f/2.8 on the 1.6X body is going to give about the same DOF as f/4 on the FF body. In practice, the EF-S 17-55/2.8 will behave rather like a hypothetical 28-88/4.5 on a FF body.

So really the choice is if you think you would prefer to have the wider FOV of the 16-35 on the FF body, or the narrower FOV (28-88 equivalent) on the 1.6X body.

FF. whats the max ISO of the 40D? it can't be more than 3200 and the noise will be heavy considering...