The only Westone IEMs I owned that have replaceable cables are the UM3X, and I have to have them replaced twice within the first year due to failed drivers and broken cable. I've never had any problems with my UM2 until after I've had them for nearly 3 years, then the cable broke and as the warranty had expired, I was lucky to have found someone who was able to fix them for me. With the Westone 4, touch wood, I'm happy to say that they are as good as as new so far since I got them nearly a year ago. But if I had the choice, I'd definitely get the RC version instead, as good and durable as the fixed cables are, they will still get broken one day and it'll very likely be after the warranty has expired. And if the cable broke near to the driver, it'll be a hard (and expensive) job to get them repaired.
Just for the record, I dislike the replaceable cable of my UM3X, as one of the drivers get detached from the cable quite easily, and i find the moldable ear rests very uncomfortable.