If you plan on doing a lot of distance photography, it'll be worth spending the money on the extra 200mm. First try to think how often you shoot at your current maximum, and how often you find yourself needing/wanting more. If it's often, it'll be worth it the investment.
As for a monopod/tripod, I personally have trouble keeping the shots still with my 300mm, so it's likely you'll need something to stabilize the shots if it's already difficult at 300mm for you.
i was waiting on a moment, but the moment never came. all the billion other moments, were just slipping all away. i must have been tripping, we're just slipping all away. just ego tripping.