the N9 hasn't got as good a camera, and may be missing other features too. All for the sake of a pull out qwerty, when you have an on screen qwerty on the n8.
The N8 is available currently, the N9 wont be out for at least 4-5 months..
If you want a phone with amazing hardware and a simple OS and arent too bothered about apps, then get the N8.
If you want apps then get the iphone4 (it does have signal issues though).
I have found the n8 to be nokia's most well built phone, the titanium shell and gorilla glass make this a well engineered phone. If you want a phone with a good camera go for the N8, if you want a gadget get the iphone.