Nikon TC's are supposed to be pretty good, but they're $$$. I think Kenco is the next step down from the Nikons. IMO it's not worth it, they're too expensive for something that is going to be relatively impossible to hold steady without a tripod.

For example a 2xTC@300mm = 600mm. 70-300 is f/5.6 at 300mm and you lose two stops with the 2x, so you're 600mm at f/11 in the best case scenario. What good is that? (Also just FYI the 70-300mm isn't listed on the compatibility list.)