Really its going to boil down to your own personal opinion and what's important to you.
For example, if you feel that really good value for your money is the most important, you'll never see better value then running your current bike until it dies, you save the cost of buying the other bike and typically lower insurance costs.
If you want as much horsepower and money is no object, but the biggest newest bike possible, throw on dual turbos, meth injection, nitros, etc.
Depending on the cost of the new bike, if you want as much horsepower as possible you might be able to get a turbo for your 08 for less money than buying the newer bike.
Like electronics? if the new bike comes with traction control and ABS then that'll work best for you.
For me personally, I'm planning on keeping my 08 for several years more as she still runs great, I'd love to have ABS and traction control but I don't think the cost of a new bike is worth it.