2x4gB set of card's are always better(slightly faster) than 1x8gB cards.
In this case you may go with the crucial card here because you are on a budget. The vengeance sports better cooling with bigger heats sinks and can be efficiently overclocked.
Oh thanks i never knew that. I guess ill just go with the 2 sticks of corsair vengeance since it will be faster and look nicer for no that much more money. If the crucial was the same but only looked worse i woulda bought it.