Focal length vs cropping

Here is my scenario. I'm headed to a new years eve concert this year and I'd like to take some nice images. The venue doesn't allow lenses longer than 6", so I'm not able to bring my 70-200. The longest lens I have under 6" is an 85LII, So I was thinking about renting a lens like the 135L. I'd consider the 200L also. My seat is fairly far away from the stage (200ft?), or else FL wouldn't be so much of a concern.

Regardless of which prime I end up with, here is my question: What is better - Using the 135L and a 7D or using the 135L with a 5DII and cropping? I get the extra megapixels on the 5DII and slightly better high-ISO images, but I get the extra length on the 7D. My initial guess is the 7D, but can someone help point me in the right direction for punching out the numbers and determining which has the real advantage in terms of pixels for the same image composition? I'd have to rent or borrow a 7D, so if the difference is nothing substantial, I don't want to go through the effort of acquiring the 7D. Thanks!

The 7D will net you 2.25 times as many pixels on target as the 5D2 (18MP, versus 8MP for a 5D2 cropped to 1.6x). It will also be cleaner at high-ISO - although the 5D2 is generally regarded as about half a stop better at high ISO, that is when you are using the whole frame. When you're cropping it to 1.6x crop, the 7D comes out around a stop better.

Awesome. Just what I needed. Thanks!