This is a question that gets asked rather often, and to be honest there isn't a simple answer to it. Some people go by the theory that Silk is for mages and Dreadweave is for warlocks. This isn't really true, since there is not "Class: _____" on them. For just about every Silk piece, you're giving up a bit of +damage, stamina, and resilience in exchange for a nice amount of +crit.
My general rule of thumb is this. If you PvE heavily and PvP on the side, absolutely get Silk (if you'll be using them in your PvE set). If you're a fire build, get Silk. If you're heavy frost PvP, get Dreadweave.
One thing to keep in mind is how Resilience will affect your choice. If you're fighting in AV and general BGs, people will generally not have high resilience, and so crit stays fairly powerful. However, resilience in arenas ramps up very quickly as you go up in ratings, and crit damage lessens. Unfortunately, even if you're doing less damage, you'll be sacrificing the same amount of other stats for having more crit on your gear.
Shatter is another thing to keep in mind (assuming you're frost). When someone is frozen, you will generally have at least a 50% chance to crit them even against a high resilience target and
with little crit on your gear. The higher your crit, the more valuable +damage becomes, and therefore the extra +damage on Dreadweave gets multiplied in value.
Extra stamina and resilience are also very valuable, especially for a mage that may not be the best geared. Dreadweave helps boost these quite a bit.
Fire generally won't go far in arenas as it is, so resilience tends to be less of a factor in BGs and lower ranked arenas (I mean absolutely no offense to fire mages btw, the spec simply mechanically isn't suited for higher arena play), and Shatter isn't around to give you that big crit chance boost anyway. This is why I'd recommend Silk for fire builds.
That being said, there are top tier frost mages that wear Silk and load up on damage. I'd like to stress that no matter what your choice is, you really can't go wrong. Both are great sets for mages, in a sense it just boils down to preference.
Live a little longer and do a bit more damage, or crit a little more for some more burst?