You try a search? This question has been asked and answered 10000+ times in the last 2 years.
I've got both and the Hyper 8 is a f--- load better thasn the Hyper 7 which is still a decent buggy, but it's getting on.
Both for racing and bashing the Hyper 8 is both better handling and stronger.

Avoid RTR though, the Macstar .28 they come with is not very good, they have rubbish AM radios and gutless servos.

Go to Ofnas or Hobaos site and rear the differences, theres quite a lot.