I'm no alarm expert, but your guy at the shop sounds like someone who just learned of rolling code alarms. That is something that most (if not all) aftermarket alarm systems tout as such a great feature. It basically rolls the frequency that arms and disarms the alarm every time you depress one of the buttons on the remote. No bid deal. This only helps if someone happens to be in the parking lot with a code grabber (big chance of that- :P ).
OEM alarm will be much less trauma to your car, as you may run into problems if it's not installed properly. IF you can find a VERY good independent shop that feels VERY confortable working on your car, go with them, you will get more for your money.
If not, go oem. Much easier, just the same money for less features.
Stellar is a company that pretty much plugs up to your prewired car, but I have no personal experience as to how good they are.