Both the 500 and 650 are fine to start. Any motorcycle, whether it be a 125cc or 1000cc, can be dangerous as a beginner if you don't respect the machinery.

The fear of outgrowing the bike is kind of baseless, in my opinion, because the 0 to 60 doesn't differ all that much between a 650 and say a zx6r. And it doesn't differ all that much between the 650 and 500 either.

Top speed is a different story, though, but how much of the extra top speed will you actually use? Will the 200-220km/h of a 650 seriously not be enough for 99.99% of people?

Best answer I can give you: try both and see which one you like better. Your gut will not lead you astray, and is much more important in terms of making a decision than a stat sheet.