Sapphire 280X (DUAL-X OC). Any good?

There is not a big difference between the two at all, personal recommendation is to go for the dualx one as it comes with bf4, and is therefore a better price deal for you, as you seem to want to buy the game anyways.

so no, if you were to play on two identical systems with these cards being the only difference, you would not be able to tell without a visual sightline on the card itself