I upgraded to the RB1 and I love it. Some complain that the nav features aren't that great, but it gets you from A-B pretty good. It may not have all the bells and whistles of other nav systems, but I usually don't care when in the Viper. Also, I love the iPod kit (so the single disc CD doesn't matter to me). I have an iPod nano dedicated to the car, so I take all the music I want with me (no need to change discs or channels). You can also control the iPod through the deck, so that's a great feature. You also get the factory look to it. I picked mine up from eBay for a decent price (about 1/2 the new price) and it came out of an 07 Towne & Country.
The aftermarket deck will give you some nice features and and all, but won't give you the factory look of the RB1, plus they may have to modify the dash or put in spacers.
It's really all up to what you are looking for.