This is an issue I inquired UE before. With the UE5c, there's more hump in the mid-bass region, giving you more active response for listening to recorded music (of course, this is built on the fact that majority of consumer has come to expect a more foward mid-bass, doesn't necessarily apply to us audiophiles on this site at all). The UE5-Pro is more flat.
Otherwise, issues such as soundstage, instrument separation and detail is really unknown. Of course, nothing in their company literature can tell you anything about those finer details.
I think another difference is that UE5c was available in full hard acrylic material only. UE5-Pro has its ear canal section made from soft acrylic material or something alike to it, I think. This material will probably also separate from the other half (the hard acrylic housing portion) over time, as had happened with my UE-10 Pro, and also used to happen with many other older IEM models from various company that used that type of construction.
I say either go with UE5c and save the money, or if you're poised to spend $200~$300 more, get the Sensaphonics 2X-S instead.