Personally, I think San Andreas is better. GTA 4 is just too watered down.
San Andreas was fun but obviously very different than GTA4. I think GTA4 really shot for realism and achieved it. For example, do you really think it's watering down because you can't hijack an ambulance and do vigilante rescue missions and get paid by... someone?
GTA4: Fun. GTASA: Just as fun, but different.
Also you can't go wrong with 4's graphics.