Apart from the fact that you make it sound as though XING is a fantastic place to work at I am also looking for a position where I can make better use of my diverse skill set. It seems as though the position of the Visual Designer would be a perfect opportunity to do so whilst continuing to further diverse and broaden my horizon as well as the XING-product-portfolio. 

I like the idea of working in a small and fast paced team within a bigger company. I have also currently applied to start-ups but I feel like XING might offer a comparable atmosphere with a much bigger company / team behind your back. It is that combination of a start-up atmosphere and an interesting and solid company behind it that make this offer all the more tempting.