Depends. Will you or will you not use thunder team? Because if you won't, then getting weapon instead of eidolon is just a waste. And as for if it is worth getting eidolon to 3 stars... I am still using Apocalypse that only have 2 stars. So it will be usable.
Besides it's not like you need to exchange it right away, the event shop is not going anywhere and you can decide it at later date.