When do I get to use my CE mounts?

[REPROBEAN_CHILD]

Is there any benefit from joining these Companies anyway (aside from the chocobo)? And from what I could see, all the mounts run at the same speed, though I could be wrong

[Niwashi]

Is there any benefit from joining these Companies anyway (aside from the chocobo)? And from what I could see, all the mounts run at the same speed, though I could be wrong

Well, while you get to choose which Grand Company to join, the fact that you join one is part of the Main Scenario, and, like any other Main Scenario Quest, you can't continue further in the game without completing it.

As for what you gain, it does sort of seem a somewhat under-developed aspect of the game. You get your chocobo from your GC. (And regardless of which mount you prefer, the mount system as a whole is unlocked when you get that chocobo, so you'll need to get it before you can ride anything else.) There is also gear available from your GC, which is never really necessary, but can at times be useful if you haven't gotten any good dungeon gear for that level range. If you get into crafting or gathering, there are supply/provision missions that can get you a significant amount of XP by turning in certain items each day to your GC. That's the biggest benefit for some players, though totally irrelevant to others, depends on whether you're interested in leveling crafting and gathering classes.

And if you're interested in PvP, the teams will be divided according to which GC you're in. (Not sure that last one qualifies as a "benefit", but it's an effect to consider.)

[REPROBEAN_CHILD]

snip

So it's essentually the same outcome no matter which one I choose? As in, being in Maelstorm is no different than being with The Immortal Flames?