The news Gitzo 3 series from a weight handling perspective should work fine. Especially since the new 600 II weighs about the same as the v1 500mm IS. I had the 800mm before the 600 II, so the greater weight of that lens and all the other stuff that bolts on was a concern. That is what swayed me to get the 4 Series. Most folks I spoke with actually recommended a 5 Series, but I did not want the added weight and girth (5 series is huge).
All I would say is, for the amount of mula you are spending for a new tripod, I would look to the future and get something that you will not have to replace (unless it breaks). If you think that one day you will want to get a RRS tripod, I'd suggest just getting one now. You might save some $$$ now by getting a 3 series on Amazon, but will loose a lot if you try selling it later to get the RRS. Buy once, not twice or three times.
You could always save your Amazon points to purchase things like a Lenscoat for the 600mm or a replacement foot, or Don Zeck front cap, etc.
Well, I guess I am not quite convinced that the RRS is enough better than the Gitzos, which are themselves very good, that it warrants paying almost double the price. The Gitzo will certainly do the job, and as I mentioned before, all of the professional or other highly skilled bird and wildlife photographers I follow seem to use Gitzo exclusively, so it can't be all that bad of a brand. I also have no doubts that the Gitzo will last me for...well, forever, barring some random unknown event that breaks it for me ( ).
On top of that, I have a bunch of Gitzo parts and cleaning supplies, such as the grease used for the joints and such, that came with my other Gitzo Series 0 Mountaineer (GT0541). I guess I also feel it would be nice to stay within the family.
Sounds good to me. I trust Gitzo and that's why I bought another. I have three Gitzos and they have never once let me down. The new version 2 of their systematic line is very innovative, you will be both impressed and very pleased. RRS I am sure is great too and I have one of their monopods which is an absolute beast.
I too purchased my Gitzo from Amazon, used my Amazon Visa points and got the Gitzo rebate to boot. From a value stand point, you cannot go wrong. Best of luck!