I'm pretty sure that with flying mounts, X > Y assuming that you skip the non-100% drop rate collection quests. I took three days of playing to finish off all the outlands quests (non-100% drop rate collection quests) from Blades Edge, Nagrand, Netherstorm, Shadowmoon, and some of the Nagrand quests and had enough money to buy my epic flyers. With blue flyers, I believe the gold / hour is still better or at least on par with dailies. Add to that the fact that dailies have a definite limit on them.

I used to grind 10 dailies (the easiest kill-type quests I could find) for appr. 500g per day (took around 45-55 mins depending on how busy SSO was). Even assuming that that you do that every day, you'll still have to spend almost two full months grinding dailies to get your epic mounts (52 days). You can, of course, add more daily quests, but the gold / hour will decrease. I'm pretty sure that until lich king is out, doing skipped quests is faster.

Add to that the faction bonus and team experience you get from instance grinding.

That being said, I'm on the get to 70 asap wagon - we don't know what will happen to the value of gold in the expansion - most likely it will drop just as it did when BC came out and paying for your epic riding skill will be much more of a breeze.