No, I've never had the opportunity to hear the M50W. Yet. I say "yet" because one of my neighbors might be getting a set, and if they do I'll be able to hear them in person.
It's a husband and wife. He's been playing the bass guitar in local bar bands for over 30 years. She's a bit of an "appearance" type of person, so style means a lot to her. When I got the M20's I had them over for a listen, and they both fell in love; he for the musical clarity, she for the gorgeous cabinetry. However, they needed something a bit bigger, so in late November of last year they ordered the M50W instead of the M20W (from the same place I purchased them from, Trax Audio). Trax had none left at that point, but they were in the process of placing a big order with Swans so they were going to add a few sets to keep in stock.
My neighbor emailed Trax just this week because she hadn't gotten them yet (they were supposed to be available by the end of January). They said there was a problem with the shipment and it would be at least another 3 months, so she cancelled the order. I found that The Audio Insider was now stocking the M50W, which they weren't doing last year, so she's going to contact those folks.
If The Audio Insider does have them she'll probably get a set. If that happens then I can hear them with my own 2 ears and will be able to answer your question definitively. However, I can probably give some advice based upon my own experience.
The M20W sounds great, no question. It could use just a touch more on the top and bottom ends though. Nothing critical mind you, but probably a little to finish out the sound. The M50W adds a tweeter, so the top end should be taken care of. For the bottom end the M50W's subwoofer speaker is bigger, the cabinet is larger, the amp more powerful and it has a port. I can only imagine the differences are quite pronounced.
Having a bit of a discerning ear myself means I tend towards quality components whenever the wallet allows. What I get from the M20W's is rather impressive, so I believe the M50W will be impressive+. Personally, I can't see any downside to the larger system, budget permitting of course.