Brianmgrarcom - I ended up with both of these Harbeth models in part because I found it hard to judge the difference between the two without hearing them, and I really wanted to hear them (which I couldn't very easily down here in Texas). That having been said, however, in the last year or so, Paul Szabady did separate reviews of all but the Monitor 40 for Stereo Times, on-line. Bob Neill also did a comparison of the Monitor 30 and the older Compact 7 for Enjoy the Music (March 2002). I think both writers (and I know neither) give basically truthful and accurate accounts. Of course, being different people with different ears and different equipment, their conclusions are somewhat different, but they go in a roughly similar direction.
In short, some people would find the Monitor 30's to be a bit too revealing for their tastes. (Neill calls it "piquant," though my recollection is that he had a slight preference for the M30s, nevertheless.) Szabady seemed frustrated with the M30's...they were hard to match, equipment-wise, and tended to tell him more than he wanted to know about certain less-than-perfect recordings (which is an issue I can relate to heartily).
However, the M30's have a really great soft dome tweeter, same as in the flagship M40. When it is playing something it likes, it is heavenly (in my experience). With an equipment and/or source material mismatch, however, it can indeed tell you more than you want to know.
The new Compact 7 is a bit more forgiving than the M30. It is still sensitive to equipment that you use with it, and you can even cause it to have edgy highs (and what speaker can't?), but it is mostly pretty all around satisfying.
But the M30...all I can say is, when it is ON, it is nothing short of sexy. (As in, try some female vocals with it. Or, for that matter, male voices, come to think of it. Harbeths are justifiably famous for what they do with voices. I know of few speakers that sound better, or even as good.) (Also, if you're a classical listener, I think Harbeths do strings better than any speaker I've spent time with, although I don't doubt that Quads can give them a real run for the money.)
So, if it sounds like a close race between the two, I can't say much...I still have both pairs, and the longer I've had them, the less I've felt like getting rid of either. (Fortunately, they're small enough they're not that hard to switch back and forth.)
Hope that helps.