Few desktop environments have benefited from the recent diversity of interfaces more than Mate and Xfce.
A year ago, Mate hadn’t even reached general release. However, since then, it has been influential in making Linux Mint the distribution of choice among experienced users. Similarly, after years of being the third most popular desktop environment, Xfce has become one of the major alternatives.
However, despite their similarities, which one is likely to appeal to you depends on what you are looking for in an interface.