Normally, but not always, one first class seat is cheaper than two economy seats. And, given the choice, I'd rather have a first class seat than two economy seats. Additionally, (1) it'd be an easier, more straightforward booking, not subject to something going awry, (2) you'd get the perqs of first class - free checked baggage, fast security line, early boarding, enhanced on-board service (meals, drinks, snacks, etc.).