The Fujifilm X-S1 camera takes a standard SD card, but it can also accept a micro-SD card in an adapter. I have used a number of different brands SD and micro-SD cards, and have not experienced any difficulties. My preferred brand is Sandisk, but it works equally well with some Lidl and other supermarket 'no-name' brand cards as well.
I suggest 16gb Sandisk Extreme SDHC UHS-1 high speed SD Card for your XS-1 good for photos & HD movies. Buy 2if you can and then you have always got a spare if you fill the other one on your photo travels. You can then carry on shooting & download the other one when you can. Hope this helps.
It uses standard size SD cards (or micro SD if you use the appropriate adapter, but there is no real point in doing this). Aim to use SDHC cards for faster transfer speeds. I am currently using Fuji SDHC or Sandisk SDHC class 10 cards (standard size), with decent results for stills and video. As per Chris Graylings response the Sandisk cards are a very good bet for reliability. The 16GB cards are a reasonable compromise between storage space, cost and the irritation of losing photos or video if a particular card does malfunction. You can therefore afford to have a few available.