I can't comment on the 5.4's, having not heard them, so I can't really answer your question. However, I do own the C2's and I highly endorse them. I'll admit to being a big Dyn fan, so I didn't listen to a lot of other speakers in the same price range, but if you're looking at these two, presumably you are a fan as well. They're probably both overpriced at full retail.
Even at a 50% premium, if you're looking for music (rather than HT) performance, the C2's will be a much better answer IMO. I find the full range to be exceptional - tight, controlled, transparent, and able to go as loud as any sane person would want in my medium-sized room.
Sorry I can't comment on the 5.4's - I'm sure they're excellent speakers too.