Different venues. Snakepit was the old place but is still used sometimes for trainings which is why you might be thinking it's used interchangeably.
Sandpit is the WEC which is right on the beach and actually a pretty cool spot with lots of food options within a block or two. There's even a brewery/nightspot under the same roof, though you have to exit the building and walk around to the other side to get in.
If you ever want to see your non-Hawks team play on the road, Wollongong would probably be a good spot to do it as things are quite accessible. Fly in to Sydney, train down to Wollongong and then pick accommodation within walking distance of the venue. Good beaches and so on all easy to get to from there.