Two very interesting choices! I have the Harbeth P3ESR at home and have owned a bigger Dynaudio model previously (Dynaudio Focus 160). The Focus 160 is a fantastic speaker, if you have a very powerful amp and quite a bit of space to let them breathe, otherwise the bass will be too much in a small room (IMO). The soundstage is huge and the overall sound is extremely pleasing, you can listen to music all day long! The smaller X12's are is supposed to be easier to drive and doesn't require as much space. However, I am 100% sure the Harbeth's are much easier for the amp than any Dynaudio spekaer! They are very neutral in a good way and for me never boring, sounds right with every record I play, and the soundstage is big too, and very easy to place and doesn't boom or anything like that. Very good with voices on movies, tv, radio etc.
Personally I would pcik the Harbeth's, as I don't need to worry about space, lots of power and they sound fantastic with the music I play. They are very small and nice to look at too, I have them in rosewood.
The Dynaudio's should be a great choice too and much cheaper.