Hi there! I've been playing Minas Tirith for a while now and agree with what has been said above. One of the guys in my local group has created a fantastic guidebook for playing as this army. If you want to PM me your email address I can send it to you.
Additionally, you may have seen the experimental army bonuses created by the middle earth team for Throne of Skulls (if the army is chosen 100% from one faction). MT's is great - every model gets +1 courage!
Gandalf the Grey is not in the army list, although Gandalf the White now is (according to these experimental rules, which will almost certainly appear in the new Middle Earth SBG next year). My point is, take white over grey with minas tirith!
Some final tips: faramir or boromir on horse with lance, leading knights is great as a heavy hitter unit. You need a battleline of at least defence 6, with fight 4 spear support (you can max out the 33% bow limit with rangers, then fill out the rest of spear support spaces with citadel guard with spears (both the same points value). Sprinkle fountain court guard with shield across the battleline for that D7, mmm. Useful heroes aside from damrod/beregond are Denethor and Madril. Denny is a beast in combat, and more reliable with the new army bonus, and madril comes with 3 might, and is a beast at range or in combat considering he costs the same as a captain with a bow! ( he also has a great special rule).
Have a great time