I quite liked the C5, it has amazing bass and pretty darn nice mids and treble. I took mine back because of some pain issues and the fact I would never be able to get the MIC to work with my winphone. But the UE900 is a significant jump in all areas except bass, but I would have accepted nothing less considering the price difference.
I am happy with the UE900 but that is because I like it's treble and midrange presentation but others will appreciate the Westone signature more. Neither is "better" but they will be "better" for each individual. Get the one that's best for you...
I think that the C5 sounds too warm and laid back, but they are very forgiving. I actually don't have an issue with the C5 ergonomics; they fit quite nicely for me, and are quite comfortable. I really like the design and build quality (cable is still, though). Even with Comply tips, however, they don't isolate very well (and have a semi-open back). But once you've heard a really great IEM, there's no going back. I'm more into detail and space, with great mids, and those are not areas where the C5 excels.
I'm really hoping that the UE900's open up somehow. So far, I'm a little underwhelmed. They sound very good (although lighter on the bass than I would prefer), but I was really expecting to be blown away. The ergonomics could be better, especially the swiveling cable (which is really hard to understand as an issue until you have to deal with it...). The nozzle is on the bigger side; they fit fine with the small size Comply tips, but those are pretty small with not a lot of material. Clearly, getting them positioned right is important, and I'm working on that.
I've got two weeks to give them a try. They are still much cheaper than even a single CIEM from Ultimate Ears (a single 18 Pro, which is a six-driver CIEM, would be twice the cost; the 11 Pro 4-BA model would still be several hundred more), and they do have the nicest remote cable of the high-end universal IEMs.