I think the consensus is that Yatagarasu is the better option for eidolons. Weapons... I'm not sure, I'd grab both if I could. Prioritise MLB for eidolon, weapons... can wait unless you see yourself/ already maining/ secondarying the corresponding element.
The shop actually has 5 eidolons and 4 weapons in stock, enough for you to max limit break the event specific weapon/ eidolon.