Thank you to all that responded. I am learning a ton from you guys, thank you for sharing so much information. Alaska is a bucket list trip for me and I will certainly take all of your advise if I ever get there.

I am still on the fence with regard to the 500mm vs 600mm issue. I do shoot birds whenever they present themselves in the local woods and parks, however I also am always looking out for whatever else shows up. Within a few miles of my house we occasionally see small birds (cardinals, titmouse, various woodpeckers, etc.), ducks (mallard, wood ducks, large birds (gbh, egret, red tail hawks), red fox, whitetail deer, etc. For the past few years we've been trying to visit a different national park (US and Canada) whenever possible. Typically once or twice a year. On every one of these trips (except the recent RMNP visit with the 600), I found myself wanting a longer lens to capture the wildlife.. Could have been shooting from the car, on the side of the road or more often out in the woods on or near a marked trail searching for something to see.

Over the past ten years I've purchased and still own (for now) a Canon 100-400 II and a Canon 400 f5.6 (and some shorter lenses). While these lenses are relatively light and very easy to handle, I was never really impressed with their performance for long distance photography. Maybe I was just expecting too much, but I rarely got photos I considered wall hangers, unless the subject was at or less than about twenty-five feet away. Fast forward to a month ago, I finally broke down and rented a Canon 600mm II along with a Wimberly gimbal head for a two day trip to RMNP. I immediately fell in love with the 600. It was razor sharp right out of the shipping box, even with my 1.4 III extender. To my eyes this 600 II was sharper with the 2.0 III extender than my 100-400II or my 400 f5.6 without extender. I've been reading about the Canon super-telephotos for years and I've learned firsthand that all of the hype is definitely true!

I've attempted to follow the prices of the 500II and the 600II for a while and I am pretty sure the 600II dropped significantly when the 600III became available. Since I don't see myself purchasing the 600III in the next five years I don't consider it a consideration. I gather about the only thing "better" about the 600III is the weight savings.

After closely reading anything Bryan reviews and comments on for years and all the information you people shared, I am leaning towards the 600 II, especially since there is 'only' a $500 difference. I plan to purchase a Wimberly gimbal (or whatever you guys suggest) and mount it to my mono-pod most of time, did this in RMNP most of the time. If I think it's going to be longer shoot I will likely break out my tripod and swap the gimbal over, I did this in RMNP when shooting Elk as the sun was setting from about 500 yards away with surprisingly good results. I suspect the only time I'll be hand holding is if I'm within about fifty or 100 feet of the vehicle. I'm 6'2" and 220 pounds, 52 years old and do physical work often, but I know my limits.

I'm thinking I'll have the 600II mounted to the 5DSR on the monopod hanging over my shoulder and the 70-200IIIS or 100-400II mounted to the 5DIII hanging off my rapid strap. That should cover all of the seagulls, non-migratory geese (they never leave), stray cats and dogs on leashes at the local park that I'll likely see once I spend all of mt retirement money on this new gear...