I second spuffock in saying that prototyping is better, most simulators work under ideal conditions, when you get to the real thing, sometimes strange things happen that the simulator didnt simulate, like noise.
Some of the new more expensive simulators like MultiSim are getting better at simulating non-ideal components though.
But alas, prototyping can get expensive, so I recommend a suite like Proteus with the ISIS Simulator, simply because it supported simulating quite a few different PIC Microcontrollers, so I could write code and simulate. Which is handy. It also has the ARES PCB design software.