Larger cassette options on a compact set up? I've been thinking of getting a larger cassette for my bike to tackle hills better, and I'm just looking for some thoughts.

I have a 2011 Honky Tonk (http://www.evanscycles.com/products/kona/honky-tonk-2011-road-bike-ec026518), with an IRD 12-28 on the back (the IRD is kind of meh, shifts rough).The setup is pretty middle of the road components wise (Sora on the front, Tiagra on the back, 9 speed). 

Harris Cyclery offers a 13-30t cassette. Which would fit my current derailleur. But if I wanted to get something like a 13-34t cassette which long cage derailleur would people suggest swapping to?
This ships with a Tiagra SS. 

Derailleurs come in 3 lengths: SS (narrow range) , GS (medium range), SGS (wide range). 

If you want to move past 30t, you need to switch to a SGS derailleur, likely from the Shimano mountain range (e.g. Deore M591) - its 9 speed so the cable pulls are the same between the 9 speed deore and 9 speed Tiagra. This is what I'd likely do if i wanted a bigger cassette.