Definitely sand, especially for a tank that size. I have gravel in my 225 gallon and I hate it, it is a complete PITA to clean and I can stand there for hours (literally) siphoning the crap out and never get it completely clean (or even close to it). With sand, all of the poop just sits on top of the substrate where it is very easy siphon out. Plus, sand looks a lot better.