A 18" frame is tiny for someone 6'3". Perhaps you haven't ever known the joy of riding a bike that's the right size? Test ride both the 19.5 and the 21. If you like the 19.5 better, I agree with Rocco, and you should buy it.

If you like the 21 better but were previously set on the 19.5 because of a sale, limited inventory, buying a particular bike secondhand, etc ... um, I wouldn't do that.