You really want to avoid using a Crossfire configuration when you're upgrading. Unless you already have an R9 270X, don't buy it. It will perform better than a single R9 280X, but it will add unwanted power consumption and heat output. Not to mention, microstuttering and inconsistent optimization in games will make it a bad investment if you decide to opt that route at first.
The best configuration is a single, powerful card. Crossfire and SLI are best used when you want to make an economic upgrade. For example, I have a Radeon HD 7870 and recently bought another one for $150. I'll be adding that into my system and running both of them in Crossfire. That's how you should do it. Buy an R9 280X now, and then later on, rather than trying to sell it second-hand and buy a more powerful card, just buy another R9 280X.
If it was strictly between those two cards I'd get the Sapphire R9 270 just to avoid the Gigabyte cooler. I've seen too many issues (of various other people) with the Windforce cooler to trust it anymore.
R9 270 Crossfire will definitely outperform a 280x, but as Novuake said it practically stops you from further upgrade without throwing both cards away. Crossfire is also not perfect, many games don't use it or don't give much performance for the second card.