i have been asked this question many times from my guild. the answer i always give is that you should pick the ride that you can afford in a reasonable amount of time.
then focus back on your runes. runes have a limit and once the runes are on 12, you have nothing to spend your coins on anyways except for a blacksmith once in a while. so its important to find a balance. you also have to realize that after a certain time (stars, feeds, xp) on the mount its nearly impossible to level it up unless you push thousands of gems into it.