The Preppy is the best for first timers. Platinum nibs are conistent and the cartridge/converter system is the most veritile of all the filling systems. It's a good baseline to gauge what you prefer.
I just can't recommend springing for the converter though. Platinum converters cost more than the Preppy. Their cartridges are sturdy and can be reused if you clean them out. I recommend picking up a blunt tipped syringe and refilling them with ink samples or from bottles.