300, if you have the choice. there is no replacement for cubes for power. However 250 isn´t bad in it´s own standing.
Now, if you are running desert races or long distances between fillups, I would go 250 or even 200. once I had a 490 and it drank a tank faster than all get out. 300´s are about 50 miles/tank if you are easy on it. Maybe 25 if you are racing it.