I have an M26S and they have water ballast. It is on the tender side but it is lighter and faster than most boats in its class (size and build quality). The 26S is not the motor sailor hybrid like the M and X, and as such is a more traditional sailboat and handles accordingly. And it does trailer very well. I regularly pull mine with a V8 Envoy (smallish SUV) That said, the only advantage of the water ballast I know of is that towability. As Faster mentions, without elaborate transfer systems, I don't think water ballast can be quite as effective as fixed ballast. The other thing it does is take up headroom space as you are, of course, standing on the ballast tank. Other than the headroom, the 26S is one of the roomiest 26 footer you will find. BTW, the swing keel is not weighted on this model, so practically all of the ballast is water. So my short answer to your question is you can get an average sailing boat, with above average room below, below average headroom and excellent towability by going with a water ballasted boat like my 26S.
S/V Boccata d'Aria
I'm not sure what Dickens are, but I think they may be important and I sure as hell don't want them scared out of me.......Izzy