If you're planning on playing for a long time, buying the mount and transfering is the better option. You can always put coins in runes later, but the more you wait tog et a new mount, the more you lose upon transfering (either gems for a perfect transfer, or experience for a partial transfer).

The best option is the following: if you plan on getting a new mount, save another 2B and get the aurora chimera. That's your best choice in the long run. You'll think it is when all your runes are level 12 and you have nothing else to power up.