SleeN has more control over his gfx as always, whereas Xtacy was willing to to try something more than what a typical signature would offer, it seems.
In terms of creativity, Xtacy was more risk-taking, though it wasn't exactly executed well.
Sleen was loyal to his style of his gfx consisting of one major colour scheme. This isn't necessarily bad though, as his style is admirable, and while it is always better to try something new, he doesn't necessarily have to as it looks great.
To be honest, I think both gfx could have been done much better; the use of C4Ds are mediocre in both Xtacy's and Sleen's works, which really hurts the quality of the whole gfx as it takes a lot of the spotlight.
I don't really want to give a vote as I think both signatures has their own unique strengths and weaknesses, but in terms of overall aesthetics, I would have to give the vote to SleeN.