Unless you have a rear clam hinge it might be a coin toss as to which one would be easiest to troubleshoot. Unless you have a rear clam hinge you'll probably have to jack the car and pull the engine access panel to check to see if the starter is getting a signal from the ignition switch. To access the immobilizer you'll have to pull the seats and rear speaker panel to gain access to the immobilizer. Neither is particularly hard, just a little time-consuming. Let me know how it goes, I've got my interior gutted and the front and rear clams are off so I have good visibility if it might help.
Going over the basics again before this, what voltage should I be getting at the ignition switch?