Nowadays at least in my area Expeditions and Navigators are running about the same price. The Armada will give you more get up and go due to the bigger engine size, power & tq numbers, but I personally feel the interior leaves a lot to be desired. Expedition and Lincoln interiors are nice with slight variations in design. Think of it as more as different elevation choices you get when looking at a house then anything revolutionary. The differences seem to come in seat surfaces, gauge cluster coloring (white backlit instead of green), HIDs for the Navi, different audio components, etc etc. So far have put about 65k miles on the truck and have enjoyed owning it (15 in city and 19 hwy) and don't have any plans of getting rid of it anytime soon. Biggest issues have come as of recently which are as follows:

bad alternator
bad grooved idler pulley making an awful noise
sometimes temperamental deployable running boards

Other then that so far everything a ok.