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based on everything i have seen, and if you have millions of pages. you cannot go wrong with Distilled. Major respect for them! They know their stuff, and they are super good at preaching what they do as well. which means what they do must  be the cream of the crop. just my 2 cents, really do not know very many of them, but I can tell you these guys are true pros.

I agree with Chris. With such large websites it would be advisable having a sitemap index and then splitting the index into various individual indexes such as Pages, Products, Categories, images, media, tags etc.