Just some general thoughts:
I feel that Warriors of Minas Tirith (the core troop of a Minas Tirith army) are a bit of a gamble. You can rely on them to grind down enemy models that are Strength 3 (such as Orc and Haradrim), but they tend to die too quickly to Strength 4 (such as Morannon Orcs and Uruk-hai).
Minas Tirith has a very good selection of Heroes, ranging from cheap options like Cirion to more powerful Heroes such as Aragorn and Boromir.
Knights of Minas Tirth are, in my opinion, the best cavalry in the game. They're reasonably cheap, pretty solid, and hit hard.
The thing that I feel Minas Tirith lacks most is good elites; yes, they have Guards of the Fountain Court but I'd like something with Strength 4 or two Attacks.
Mordor is a very versatile list, probably the most versatile of any army. You can create an elite army based on Morannon Orcs, or you can go for numbers with normal Orcs.
I think Mordor can be a little more specialist than Gondor. The tactics of a Gondor army aren't usually much more complex than "form a shieldwall, and march forward". On the other hand, there are some models in a Mordor army that you have to be careful with (examples include Nazgul, Trolls, and Shelob).
Like I said, just some thoughts.