I say GO FOR GOLD. As a few mentioned before, the difference between GM and FO is huge!
You also mentioned getting a credit card--there are several sign up offers that allow you to get 10k MQMs with the Platinum Delta American Express Card.
In addition to having Sky Priority and a better chance of upgrades as GM, I think the 100% mileage bonus vs. the 25% FO mileage bonus well help offset any doubts of paying a bit extra for tickets, or getting a credit card to ensure Gold.