If you squash the few bugs that seem to be common with these vehicles and find one that has been properly maintained, they are pretty reliable. Plenty of folks getting 250,000+ miles out of these. Even some up in the 400,000 range. My '07 Armada LE currently has 117,000 on it and is running great. Probably better than when it was new, due to upgrades.
With the Armada being 4 years old How many miles do you have on your Mada"? I just turned 90,000 on my 04 SE.:)
For your choice of 2005 or 2008, I'd be more inclined to go with the '08, but again the previous owner maintenance matters. Before I would make the purchase, I would want to take it to a mechanic and have him go over it. Particularly looking at suspension components (struts, ball joints, brakes, rear leveling if it applies), underbody appearance for rust and exhaust system, definitely have the exhaust manifolds checked for cracks (known issue on these), have the radiator checked for leaks especially around the base of it.
Just try and get a handle on what you might have to put into it to bring it up to speed before you buy it.
(If cracked exhaust manifolds are a $2,500+ fix)
Also get the VIN number and run it here to see if the recalls have been addressed:
Use our VIN lookup tool to check for recalls on your car, or search by make and model. Also, get recall information on car seats, tires and equipment.