Critical hits deal 25% extra damage (raw only, no elemental). So at 100% upkeep, 300 raw attack effectively becomes 412. I'm not extremely savvy about damage calculations either, and they get complicated and scenario-dependent, but with a comparison this lopsided the elemental option would have to hit a very, very, very, very juicy hit zone on an ultra-weak monster to come even close to performing as well as the raw weapon.
One thing, though, since armor budget is limited, it's usually not worth pumping affinity up to max before investing in alternative damage skills like Critical Boost or Attack Boost.