From my perspective, the stats on the Eidolon are more important.
I don't run a team with Light or Thunder, so it's not that I'd need her for my "main" slot. But a fully uncapped SSR Eidolon in the sub slots is going to give a lot more stats than the alternative, as otherwise you're just using material exchange Eidolons or praying to the jewel gacha every few weeks.
Comparatively, for weapons I can easily farm the Disasters every single day until I have a proper set for my team. They aren't endgame weapons, but we're not anywhere near endgame content. However, between now and then they'll keep my squad more than competitive.
Unless one has a Light-based squad or they plan to use the Bow-based Soul (when it comes out), I don't see the point in going Weapon over Eidolon.
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The difference isn't really health, just damage. If you have a way to avoid/heal around it or you can just zerg through her it shouldn't be a problem at all.
My team with 10k Attack was doing Standard originally because I assumed I couldn't do Expert yet, then I just had extra stamina to blow before a level up and I went for it. I lost most of my team, but it wasn't especially worrisome at any given point. I just blew all my summons at every opportunity and kept a close eye on my skill rotations and it went fine.