Generally speaking, one fast card is better long term. You can add a second card later if you decide you need more power. And later on, that second card in theory should be cheaper than the original.
Crossfire & SLI also have the limitation of giving you performance gains based on how well optimized the game or drivers are. So the rate of return isn't always the same.
Two cards will likely create more heat overall, and consume more energy. If either of those concern you at all.
This is true! A single heavy duty card is always better than a CF setup upfront. Go for a CF setup later when you need it. Its not that a person needs 100+ FPS in every game.
A CF setup has heat/power/air-flow and arrangement consideration which can be cumbersome!