Its a good question, and you will decide once you get the lens.
I usually analyze my potential purchases to death by looking at apertures and focal lengths I use, and asking myself what benefit I'd get from the new lens. Sometimes I'm wrong, but I usually buy used if there is a question, and often resell a unused lens for more than it cost.
If there is any question, rent first. I seldom used my 300mm f/4, and a f/2.8 would only get used because I'd feel obligated after spending so many $$.
I recently bought a used Nikon 200-400mm f/4 VRI, but have never seriously used it. However, it only cost me $2500, and is mint! I can sell it for almost double my purchase price any time.