It's a good question. Personally I'd say get a MLB version of Tiamat as a priority. That's 600 x 5 = 3000 orbs? She's got very high base attack and a decent special (barrier to all) so she's basically in all my teams. I'd recommend saving the 10K to get a Dragoon in your preferred element, but that's a long-term investment. Completing other eidolons may be worth it, particularly if you have a gap - either need the power, or need that element and the attack is still good for you. I'm now trying to decide between saving for a 3rd (different element) dragoon, or burning the orbs on draconic eye shards.