Shimano sora 9 Speed rear derailleur capacity It's 37T here http://bike.shimano.com/content/sac-bike/en/home/components11/road/sora1.html

But 31T here
http://www.wiggle.co.uk/shimano-sora-9-speed-rear-derailleur-1/#prodbullets

Why is the difference? 

I want to install a 50/34 rings with 11-28 cassette. I have the cassette and RD already but not sure if the RD has the enough 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.