Just to give you an idea, an Arnor force is more "traditional" in play, in that you'll be limited to 33% bows. The warriors have high defence and come with built-in spear support, which can be handy. They have better than average fight but worse than average courage (as far as average "Good" models go). The theme is appropriate for any time in the first half of the 3rd age.
Grey Company is quite different, with low defence, but every model can shoot (if you take the required apportions of one Ranger of the North for every 4 Rangers of Arnor). The theme is appropriate for any time after the fall of Arnor. GC can be quite tedious to play against, because the first tactic people tend to latch on to is running forward within shooting range, then backing up every turn. Don't be surprised if your opponent runs out and confronts you with the Shadowlord the very next game
GC is fun to play in balanced scenarios, as allies (say, with Gondor), and if you're not afraid to get them dirty with some hand-to-hand combat.