Am I going to notice a difference in I.Q and high ISO noise DR etc with D3100 or would I be better going FF e.g. D600,D700 as an upgrade for a real difference? Is it going to be worth upgrading from a D3100 ?
You won't notice a massive amount of difference. You'd be best spending the amount on some good glass, however if you need the tougher build and pro orientated ergonomics, 6FPS etc etc then yes it would be worth it. Going Full Frame boasts a huge improvement in low light quality but requires an investment into FX lenses to gain the best of the cameras abilities so it's not a cheap outlay!