I play a LM. I used a heavy and a medium steed. The heavy steed is a tank. I didn't try the light steed, so I can't say much about it. The medium steed is a good default choice. After you have gotten some mount levels under your belt, spent some points on maneuverability, and gotten some time in the saddle; then you can see which type will work best for you. I never did get to level 50 in the last couple of beta rounds. Max talent points was 77, which we will see if that holds on release. With 77 points, you could not max out everything (3 discipline trait lines and five main stats). You will have some build choices to make.
"No sadder words of tongue or pen are the words: 'Might have been'." -- John Greenleaf Whittier
"Do or do not. There is no try." -- Yoda
Indeed, in a world and life full of change, the only constant is human nature (A is A, after all :P).
We old vets need to keep in mind those who come after us.