I have listened to the Rennaisance's before. They are very impressive; and very large. Ultimately, your room size should be the main deciding factor between the Rennaisance and the Expression. In a nutshell....
If you have a fairly large room, and if you sit a bit further back, and if you want the maximum depth in soundstage/image, and if you desire that slightly deeper bass response, and if you need that extra 1 db of efficiency, AND if you have the cash.... then consider the Rennaisance.
If you have a moderate to medium sized room, and if you sit a bit closer, and if 24hz is deep enough, and if you don't mind the slightly lesser efficiency, and you want to save that extra $10,000.... then consider the Expression.
Also, bear in mind that the ARC technology is highly effective and is available in both of these models. It really helps to iron out a lot of the acoustical and room placement issues common to electrostats. I saw a demo of this technology performed by a ML rep at a very popular dealer in Canada. I couldn't believe the difference!