The 970 does boast a good performance improvement over the 280X. So it's dependant on what you want to achieve, and what the rest of your system can handle.
So yes, depending on the rest of your system, it holds a very decent performance improvement. AMDs typically run hotter, ergo tend to generate more noise depending on it's cooling system. But this is more down to the relevant manufacturer (MSI, GA, ASUS) as opposed to the Graphic chip owner.
£120 is a decent price used, if he's a friend and you can negotiate a bit more of a discount, and the rest of your system can handle it easily enough, I can't see why not.