The other responses are correct for the current state of the game. But just like how wind now has Hanuman for high powered scaling, thunder will eventually have Nidhoggr available (in late May). What this means:
1. You don't need to spend orbs to pick up Amaru from the exchange just yet. But should you ever roll Nidhoggr and still want a filler thunder eidolon for stats, that's when you go get Amaru.
2. If you're capable of easily picking up future event eidolons, do so. Just in case you roll any of the new scaling eidolons. Most future event eidolons have pretty good base stats (Python's weak at 1290 atk and Abou's the next weakest at 1770; 3rd weakest is...Stolas at 1860, but +10% makes her 2046 for wind teams).