The 5D MkII is said to be a tad (small tad) better in low light but in a practical sense it's not much.
The T2i has been dropping a bit in price with the news of newer releases so it's becoming a fair value, $769 today at B&H. There will, of course, be other expenses but the separate audio recorder may not be necessary if the Magic Lantern firmware proves stable (reports indicate with ML you can turn off AGC and effect manual control of recording levels). And the T2i does have some improvements over the 5D MkII
Just some things to think on.