I borrowed a 5DSR from Canon last week for 8 days of mostly setup shooting. And I just received my 1Dx2 yesterday. I will likely be purchasing a 5DSR based on my experience with it. The pixel count can't be beat when photographing small birds, and the AF is extremely accurate. The body (without a grip) weighs substantially less than a 1Dx2. But images get noisy at ISOs above 800. The frame rate is fairly anemic. And the file sizes are huge. You'll need big CF cards and plenty of storage to accommodate a steady diet of 5DSR images. The other downside of the 5DSR is that image review and processing takes forever due to the large file size.

IMO the 1Dx2 is by far the more versatile of the two bodies. If you're planning on doing mostly setup photography I'd definitely go for the 5DSR, but if you want versatility the 1Dx2 is the way to go.