I honestly love the M14. It's pure sexiness is just overwhelming.
However, I'd recommend shooting for the Gatling Gun Gold and/or the Scar RD crates since they're only available temporarily. It's like Foamposites, they only get released once and then become deadstock, thus skyrocketing the price.
tl;dr: Go for the Gatling Gun Gold and/or Scar RD crates since they're available temporarily. After they remove, it'll go deadstock and become ultra rare.