There are advantages to both cameras. The 1Ds Mark 2 had good battery life, about the same as the 5D2. You'll get slightly less pixels and similar ISO performance on the low end. The 5D2 performs better at higher ISO. Both have about the same framerate. The 1Ds2 has a full pro body with vertical controls but it is substantially heavier and bulkier.
The advantage of the 1Ds2 is it is weather proof while the 5D2 is notorious for having issues in even marginal weather. With my 1Ds2, I could shoot through a rain storm and not stop and not worry about the camera. I would just let the rain pour off the camera and no issues ever. The 5D2 would die a quick death if I did that. Even taking a 5D2 from cold/moist conditions into a hot car quickly might cause issues. The shutter on the 1Ds2 should last a tad longer and the camera will be more robust overall.
The 5D2 has bad autofocus. There is no denying this. The 5D2 autofocus will be similar to your rebel and is touted as being very bad compared to other cameras. The 1Ds2 blows the 5D2 autofocus out of the water and i can follow owls flying at me in busy habitat. Focus is solid and accurate. The 1Ds2 screen is small and not great but I don't rely on the back screen a lot while shooting while others do. The 5D2 has a nice big screen with good resolution, can take movies, and has simple single handed operation. The 1Ds2 has 2 handed operation and can be a bit frustrating to use until you learn the 2 handed operation technique. While this may seem bad, I've never had my 1Ds2 change settings on me while hiking or accidentally. The 5D2 can be bumped off in the settings easily.
There are advantages to both. You have to decide which advantages you want. I push my equipment hard so for me, a pro body is essential these days. Others have other factors that are more important to them. I upgraded to the 1Ds Mark 3 and it has is the best camera I've ever used. I think if they announce a 4th version soon the 3rd will come down in price even more.
