In general, SSR eidolons will increase your strength the most in the short-term. The dual element +40% eidolons are also extremely valuable at this stage since they're best-in-class for raw damage (assuming you don't score any useful SSR eidolons from Gatcha).
Priority for new players will be (assuming MLB):
SSR eidolon (primary eidolon OR dual element +40%) > SSR assault weapon (primary element) > SSR eidolon > SSR weapon (non-primary element or non-assault)
Once you have 6 LV100 eidolons (easily achieveable in a few months of active play), new ones will provide little benefit (other than ones that will serve as a primary eidolon). On the other hand, SSR assault weapons for your primary element are limited, and it could be years before you have enough of them to fill your grid.