Also be aware that once you reach expansion areas, some older mounts aren't able to be used to the full potential of the area. Some can't fly, for instance. Something to keep in mind, I guess, since it won't really be apparent until later in story progression.

That being said, sleipnir is still one of my faves, I just haven't been able to use him as much in HW zones. So it really depends on how badly you want the mount and how much enjoyment you feel like you'll get out of it.