Renting a prime, 50mm 1.2 or 85mm 1.2

I'm second shooter at my friends wedding over the new year, and I'd like to try and rent a new lens for it, it's going to be a dark church, and the reception is in a greenhouse, so it'll be,.. welll, dark, so I'm not even sure my 2.8 lenses will be up to it.

I'm going to be starting the rental before Christmas so if I don't get along with it before the ceremony i'm not going to force myself to use it.

In short, i'm trying to decide between the 85L and the 50L, both of which get great reviews.

I'll be shooting on my 1DmkIV, with a 5DmkII as a backup, (I just hope I don't fall in love with the 85 on the 5D, that will get expensive.)

Which would you guys recommend?

As a 2nd shooter you might want more reach with 85mm.
I love the 35mm as well-great for group shots.

You can't make a bad choice with those two lenses so I wouldn't fret which one has better IQ; they are both magnificent. As Crazy suggests, let the required focal lenth be your guide and it may lead you to the 85 as long as you're equipped to shoot wide when you need to.

I'm not too concerned about image quality, perhaps just thoughts on the two lenses themselves, I'm aware for example that the 85 is a little slow to focus, I suppose I'm looking for people who have or had both to chime in and say which they prefer for wedding photography.

The 85L does require a bit of finesse because of the slow focus, but it doesn't take long to learn how to anticipate your shots a bit more with it when you need to. I think getting a feel for what aperture you can get away with at certain distances is where the learning curve is on that lens. I find 85 to be a more useful focal length than 50, which is why I stopped carrying a 50mm around, though sometimes I miss it for details.

50 1.2 is hot!

I use 50 and 85 a lot throughout the day. My two favourite lengths.

But to tell you the truth, I got a much higher keeper rate with the 50 1.4 and 85 1.8. The 50 1.2 is... well... kinda touchy on the focus, and the DOF is really small, so to me it's more of an artistic lens rather than a reliable, solid, must-get-the-shot lens like the 1.4 was. It does quite a decent job on my 40D though, but on FF with such narrow DOF you quickly can miss your target due to slight movements, or even your own breathing. The 85L exaggerated this even more, and the focus is much slower. This lens was almost infuriating to use in a dynamic, fast paced environment where you are trying to get close to capture emotions and expressions. It's focusing is somewhat decent for full length shots as you have more DOF headroom, but closer than that and I missed a lot more dynamic shots than I hit, even stopped down to 2.8. The 85 1.8 just NAILS it. It's amazing for dynamic candid shooting and can keep up with erratically moving subjects. My keeper rate on my 85 1.8 is so ridiculously high for the shallow DOF it gives.

So really, as a rental, I'd choose neither, and use the 85 1.8 instead.

As others have stated you can't go wrong with either lens. I typically mount the 35 on my full frame and the 85 on the crop. The 50 is used on either body. When I have the room I use the 50 for group shots.

I have to agree with Lloyd. The 85 1.8 is an amazing lens, if I could shoot the whole wedding with it I would.
I use it more and more every wedding. The IQ is fabulous, sharp as a tack and absolutely beautiful bokeh.....keeper rate extremely high. I did my reseach on this lens but was still surprized by the quality from such a reasonably priced lens.