Hey John!
Great question. I use Guitar Rig quite a bit; started with 2 and now have 4. When I got my Mac I switched my DAW of preference to Logic Pro 9 from Pro Tools and Reaper. I like Logic for a variety of reasons, and one of them is for the "Amp Designer" plugin that is included. There are some great tones to be found, both in their amp emulations, and in the "pedalboard" function that's offered. You can basically choose an overall tone and it will load a preset "rig", or you can choose an amp head, then a cab, then various stomp boxes to come up with something unique.
That's pretty much what Guitar Rig can do as well obviously, and I like both. Between Logic's Amp Designer and Guitar Rig 4 I have just about every guitar tone simulation I have ever needed for my recordings (which are all basically for demo purposes). Both have really nice mic placement functions that allow you to change the type of mic, the position it's in, and even the angle to the speaker cone. I know Guitar Rig even lets you put it in or out of phase, which is neat. Neither let you do multiple mics, which would be pretty cool, but hey, not bad overall. You can still mic a second cab in a different place if you want.
My favorite of all the tones from each:
Guitar Rig: their "Tweedman" (Fender Bassman) amp, which sounds really great when pushed and maybe given a little Leslie treatment
Logic Amp Designer: their "Studio Amp" (Mesa/Boogie Mark III), which is just about as versatile as the actual Boogie head. Put a "ribbon mic" on that and add their Vintage Drive pedal and you're in heaven!