I haven't used on a double rifle, but have used plenty of reddots on my zombie killing rifles. Hehehe...
My opinion is this. It all depends on your eyes and how well you can pick them up, for me color is also a concern as I'm color blind and have found that I can see the "green dot" better than the "red dot."
If you have good eyes, go with the 3.25 MOA for the reasons Uni mentioned. If you don't, or have no plans of shooting your double at anything past 50 yards then 6.5 MOA shouldn't be an issue. 6.25 MOA only covers 3.25 at 50 yards, IMO that is fine enough for that range and weapon.
I really like reddots and wouldn't hesitate to use one on a double. Check out the video I put up on my SAAM training report in the Hunting Africa section. That Kriegoff I was shooting had a docter (sp?) rd on it and it shot great. I was really pleased, especially considering I had never shot that rifle before. As Brick said on the thread, both shots would have resulted in a dead buffalo.