I have heard M3 and X5, but not in the same system. The M3 were at my brother-in-law's and driven by a musical paradise tube dac, and a prototype integrated tube amp I built. 30 watts/ch, Russian 6P3S-E tubes with 6sn7 front end. They sounded very good, and threw a huge sound stage. Not the absolute last word in micro detail and pinpoint imaging, but very good. I own X5 with external crossovers and best parts. These are driven by a Lampizator Big 7 Mk2, and the 6sn7 based line stage and a 300b push pull amp I built. That amp is 27 watts/ch. The X5 has much more micro detail and a more focused image, but it is apples to oranges because the gear in front of them is far superior to the M3 setup I heard. Still, I would say the M series will give you this cloud of sound and the X series is more focused and detailed. It throws a huge sound stage as well, but more pinpoint imaging. Both are wonderful speakers and I am not trashing the M3 ability to deliver micro detail. Bass is excellent with either and I would not consider it lacking in any way. The X4 uses a very similar passive bass setup to the M3 and I will say it is just fine. The X5 has the powered sub that produces bass from about 90 Hz down and it is fine as well.
So I think it comes down to whether you want to be bathed in this very seductive cloud of sound, with quite good imaging (M series) or you want pinpoint imaging of the AMT tweeter in the X series. Of course Clayton will give you advice on your room and which model would be better. The X series requires that your listening position be at least 7 ft from the speaker for the sound from the drivers to be integrated. That is usually not a problem for most people, but if you have a small room then the M series is probably a better choice. I sit about 10 ft from the X5 and they are 10 ft apart so they work well in my room.
Anyway, those are my experiences with the M series and X series.....but in two different rooms and systems.