Someone did a test and found shutting off your computer every night versus putting it on suspend doesn't actually save power. Maybe like 1W, which is negligible.
This is because the boot up sequence of a the computer uses a certain burst of energy, while suspend/resume uses a very small constant amount (just enough energy to keep the RAM alive basically).
I guess this would be true for desktops or laptops connected to power. If you have a portable system, maybe the math would be different, I don't know (for example, optimizing for battery life versus energy cost savings).