Originally posted by audiodoc go with the 50-500 but then I read greater reviews on the 150-500.
There has been some debate on these, and if I remember correctly the 50-500 came out ahead most times. Which was surprising given the longer focal range. Anyway if I was only going to carry one lens it would be the 50-500 assuming you are ruling out the DA*60-250 which would be my first preference. I just don't see giving up the 50 to 150 range.
I've no experience with safaris but I do understand trying to shoot wildlife at long distances, from our recent Yellowstone trip. I took the 50-500, a Pentax A 400mm f/5.6 and the DA*60-250. By far the best images were with the 60-250, second best with the 400 and third with the 50-500. Now to be fair the 400 was always used on a tripod, the 50-500 was used both free hand and on a tripod and the 60-250 was always hand held. And most of the 50-500 shots were at 500mm which is not the best on that lens. The images were OK out to maybe 425-450 but after that too soft to be usable IMHO. Had I been smarter I would have rented (or even bought) the Sigma 500mm f/4.5, I missed a lot of shots because I needed 500mm or even more. And the wildlife was there, I had the opportunity just not the lens to do it. Lot's of guys with 600mm monsters set up and my little 400mm looked like a toy.
So if you really need 400 plus hand held, then go with the 50-500 but remember that is a very heavy lens so consider some kind of carry rig. And hand held @ 500mm I don't think the IQ is there.
The question you have to ask is do you want a sharp picture @ 250 or a soft one @ 500?
Another thing to consider is that Ricoh has a 120-380ish lens on the road map. No word on when (or if) it will surface but since you have 10 months I would wait a bit and see if that comes out in time before deciding.