This is just a little note from a short time owner. The first bike I owned was a Rebel 250. I owned it for about 6 months. It was a great trainer. I am about 5'10" and the bike was not too small for me. Sure, it is a small bike, but my knees did not feel cramped nor did they hit the handle bars. I bought the bike for about the same price for which it was sold by me later. It was NO good on the freeway. 65 was the max I could get it to most of the time, and I think my 200+ weight contributed to that problem. For flying around country roads and state routes it was fine. It was a great way to learn a bike on the cheap and not lose any money when trading up when I grew out of the bike.
I think most tall people would fit this bike fine; it just looks funny because the bike "looks" so small. But I never felt awkward on the Rebel. If you could find a Honda Hawk (I think they varied from 250 to 450) they are a little bigger in size. I think a Ninja is too small for me - the 250. I sat on one and it was too cramped for me.
I think your 500 sports bike or 650 cruiser ideas are fine though. I think you won't grow out of those so fast, but conversely, I think you will not be able to sell it for close to the purchase price, unless you get a deal on a used bike.
So, if you are a huge person, the Rebel might not be great for planning freeway routes. If you are super tall, it might not fit you very well (you are not super tall at 5'11"). If you think you will be happy with the 500 or 650 for a couple years, it might be worth the investment - since you will likely grow out of these bikes, you have to decide how soon you might.
My wife and I jumped up to an 1100 Shadow Spirit (her) and a Yamaha FZ6 after the Rebel. I think it was a bit too big too early, but I also think everyone has to learn their limits and desires as they spend more time in the saddle.