Harbeth P3ESR or Monitor 30?

I wish I could give you further input, but I haven't heard the M30s, only the C7s.  Yogiboy makes a good point - you could compare them side by side, and then keep the one you prefer.  A bigger investment up front, but Harbeths sell very quickly used.  And you'd never doubt your choice if you compare them.

I have had the Compact 7s and now have the M30.1s. I have heard the SHL5s in my room. I have heard the P3s twice in a dealer audition. My room is a bit smaller than yours, 12x18. I highly recommend the M30.1, or M30 as stated in your original post (used, I presume). They should fill the room without overwhelming it. I have read some of the posts in response to your question on the Harbeth forum. It seems to me that the general opinion on HUG is go with the larger speaker due to its larger midbass driver. I agree.
In response to the above posts about the C7s' bass being muddier than the P3s, yes, that is true. But definitely not the case of the M30.1. You will get the "larger" sound of the C7 with the clarity of the P3. 

Thanks guys. A head-on comparison would be ideal, and I haven't ruled one out.

@valinar Yes, the helpful contributors on the HF, including Alan Shaw, were fairly persuasive in recommending the M30.

I have little doubt that I would be happy with either, but given that I have plenty of good power, and enough space, I do expect to be giving the M30s a try fairly soon.

The plot thickens...

I am very close to making a decision, but have received some strong, contrary opinions on the Harbeth forum. Not that the P3 isn't held in very high regard – it is. But that the M30 is a better speaker overall, and that the advantages of the other are limited.

To complicate matters, I currently have a choice of either speaker at roughly the same cost.

hmmm...

You will certainly receive contradictory opinions since everyone's taste and listening preferences are not the same. I have listened to all Harbeth speakers in the line. My least favourite speaker is the M30. No doubt the tonal quality is very rich, but the overall sound is thick and shut-in. All the other speakers P3ESR, C7ES3 and SHL5s are airier than the M30.

If you like an airy and open sound, you are better served with the P3ESR. But if you can listen to both M30 and P3ESR, that is of course the best option.

Thanks ryder. I am close to making a decision, and will update extensively when finished.