Gravel is easy to clean. It doesn't get sucked up like sand. AND fish can forage on it easily. But... it looks fake often and some people don't like the look of red, green, pink or blue gravel. You can have plants I suppose.
Sand looks most natural and stunning. It is hard to clean (or so I have heard) without sucking it up and it can make the water cloudy for a few days. BUT it can house plants very well.
Bare looks unnatural but it is easy to clean and ich cannot hit more than once. Oscars pull up plants often but you didn't specify if you were moving him to the 150. I say go for sand or get brown gravel of varying shades for a nice natural look. If you want to try something new....
sand... sand! SAND! SAAAAND!! (inside joke)