Being new at electronics, i'd have to go for g-switch. Install this end aft, fly!
EDIT: just decided to read your whole message, saw you are doing the TARC. I was in a team that did it this year, did not get to finals. We used a PML Accustage, worked great.
I would have to say G-switch, only real disadvantage I can think of is if the rocket drops off the pad and bounces at any time, it would start the timer and you would need to replace the charge or igniter.
Depends on what I'm flying. For most general purpose use, I'd have to go with a G-switch, simply because it's the easiest option (drop it in and fly). If the rocket has a low acceleration (T/W under 5:1) or is flying a Hypertek motor, I think you're better off with the breakwire, because the G-switch may not activate at the proper time. For TARC, I'd recommend just getting the one with a G-switch; your acceleration should be fine to arm the timer (check the thrust curve and the finished model weight just to be sure!).
I was thinking more of a pull pin system. A break wire sounds cool, but then I would have to replace the wire often. The G-switch sounds the easiest to use, but the switch itself seems very complex and unreliable, but I havn't touched a timer before so how would I know?
Does anyone know how to make a pull pin system? Where would I buy a switch that closes a curcuit when a pin is released?
I believe if you order with the pullpin/breakwire option, they have the circuitry built in to activate the timer when the pin is pulled or the wire is broken. One easy way to do a breakwire is to put a piece of ordinary solder across the terminals. Tie a string to this and tie the other end of the string to the launchpad. Assuming the pad has some mass, it will pull the solder apart easily on launch. Wire wrap wire works too.
I have never used a timer before. But the PET2 seems the most reliable. You can use a G-switch AND a pull pin. I think that a pull pin would be most reliable. With a g-switch you can mess the whole flight up if your motor chuffs a few times. And what if your rocket doesnt fire, then you has to risk taking if off the pad with out setting it off. But what ever floats your boat. What ever flys your rocket.