I rode my Gunnar Crosshairs a little over 200 miles in two days on a ('06 MS 150, century option both days). It was a great bike, very comfotable and no major issues. I test rode several cross bikes before going with the Gunnar. Of all of the bikes I rode, I liked the aluminum bikes the least. My top choices were the Poprad and the Crosshairs. In my limited experience, steel bikes are generally more comfotable, but this is not always the case.

I have a Specialized Stumpjumper M2 FS (mountain bike), which is a super stiff aluminum/ceramic frame that I've used three times for the MS 150. While the M2 was not nearly as nice as the Gunnar, it was acceptable. On the flip side, I have a steel commuter bike that I built out of garage sale parts. The frame is a little too small, and on rides longer than 20-25 miles it gets uncomfortable. I'd get a bike with a geometry that fits you, and worry about the frame material second.

From what you've said, it sounds like you may want to look at a touring frame.