Shimano sora 9 Speed rear derailleur capacity
There are two 9 speed Sora groups, 3400 (older, with thumb button shifters for upshifts) and 3500 (current, with a paddle for upshifts). 

The Shimano RD-3400 SS has capacity 31t and GS has capacity 37t, both with 11t smallest sprocket, 27t largest sprocket (and 16t chainring and 22t chainring difference respectively).

The Shimano RD-3500 has SS with capacity 37t (front difference &lt;=16) and GS with capacity 41t (front difference &lt;=20). The sprocket ranges have to be (11-14)t to (25-32)t for both.

Wiggle doesn't list which one they're selling, but it looks like they have part of a description from 3400 and part from 3500. 

In any case, if you were to buy one, you should get the RD-3500 SS or GS. However, in reality, you can likely use any of them (the specs the manufacturers list can be violated by a tooth or two, normally), at your own risk, of course.