I don't have Jeanne (and I won't have her leveled before the end of the event, even once I do unlock her), otherwise I'd probably try your method.
No, it's a friend eidolon. I'm using a maxed Garuda. Also, the 50% Behemoth--while indeed extremely rare and hard to obtain--is not actually that strong if you look at the sheer magnitude of the buff. It simply has the right combination of (rare) elemental buffs that fit my current team. There's plenty of other eidolons that would be stronger if I had a mono-element team for this, but my main team is water (and therefore unusable).
My run today was with a 40% Behemoth. While still pretty rare, I have more of these in my friend list, and also the buff magnitude is no more than a typical dual-element event SSR eidolon.
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Short-term, having the extra 15 levels on your eidolon will buff all your teams' stats significantly. But once you have 6 maxed SSR eidolons, that eidolon will become useless. Mid-term, a LV100 SSR weapon is better than a Disaster weapon, and it'd be useful for a thunder team (and not much else). Eventually (8-12 months?) you'd have an opportunity to max both, so whatever decision you make now won't matter.