I was also thinking about the same thing. Had a westone 1, 3 & 4 and wanted to try out the UM3x to have a direct comparison. But my W series are long gone now and I went through all kinds of threads and read up on owner's of both the UM3x and W4's comments. So, I ended up buying the Westone 4 again along with a UM3x.
I seemed happy to find out that the UM3x have really nice mids and vocal impressions that gives you the shivers. I so prefer this over my W3 tbh, as I found the W3 a lil sibilant, with the highs being a lil too sparkly. But compared to the W4, as what the previous comments said, it has a smaller soundstage. More of like, you're slightly more upfront rather in the middle of a hall. Although, I guess I wouldn't really call it a Hall in this case. Most songs, ranging from Techno, Pop, Jazz, Rock to Heavy Metal works great, but I don't believe it's an all rounder like the W4's. But it is Indeed Full Bodied and Warm and is Awesome for Gaming for those who doesn't wear Headphones or Headsets
It might not be as Bassy as the W3's but it's Just the right amount with lesser highs yet not stale and is still fun to listen to (Personal preference I guess)
As for amplification, both of em benefits from it. Amps used are the Schiit Asgard & Valhalla, M Stage Matrix, Burson HA-160D and the Woo Audio WA22 for Desktop amps while the ADL Cruise, Audinst amp-HP and an iqube V2 for portable amps. Sources are from a Sony Walkman X-1060, 4th Gen ipod, imac and a PC with a Xonar D2X attached (I do switch and use the Schiit Gungnir and the Burson sometimes). To which, I believe the W4's being Really Awesome in every Genre it was shot with and works well with all the amps I tried it out with. If you're scared that the UM3X's soundstage is too cramped, the amps helps. The soundstage got a lot wider with more air on almost all the amps, including the cheap Audinst (but not so much). Same thing goes for the W4. After comparing both of em with multiple setups, I can't help but to Really Acknowledge the W4's Versatilty, it's simply amazing in all aspects, smooth when you want it to and rough when you want it to, not to mention that it EQs a lot better than the others.
But not saying the UM3x is bad and all, it excels in Particular areas, and I'd seriously prefer the UM3x when it comes to Acoustics, Guitar solos, Jazz and Vocals. It just gives you that "Wow!" Effect that you sometimes need. And since it's Full bodied and warm, it's a lot easier to convince your friends that it's an awesome iem (Since flat and neutral sounding ones don't appeal to most of em, lol) As well as awesome with Heavy Metal and Rock. Though, I guess the bass guitar and drums kind of mix in, but easily remedied on the amps. Though, the W4 performed outstandingly Awesome on my Classical genres (Since the soundtage and better instrument separation helps here)
As for un-amped performance. It took a lil more power to drive the W4's than the UM3X. Like, i'd increase the volume by 2 or 3 clicks to have that same volume (Or say I used 50% with the UM3X, the W4's needed roughly 60%) But nevertheless, the W4 is still yet the more clearer one and still versatile towards the "Players EQ". While the UM3X is more of a Fun Sounding iem and still retains it's outstanding vocal performance.
All in all, I like both of em. I'd still prefer any of these two over the W3 though, lol. To sum it up, if you lean towards the Bassier, more Mid Forward and Fun side. Pick the UM3X. If you lean towards Clarity, Neutral Sounding, more into Monitoring (with the better soundstage and separation) Pick the W4.
Eitherway, you can't really go wrong with both. That is ofcourse, my personal opinion. My Sincerest of apologies if I missed anything out or things seemed to be too confusing. Will try to summarise it next time
*Note- The setups are not mine but are my cousins. As mine are gone XD If you read my thread before. So, I kinda stayed over and did the testing along with him*