I have been a dealer (when I was in the hifi bussiness) for both makers, sold both models and had both models. .
If you like this kind of sound they are difficult to say which of the two is better. In way word the Spendor is more lirical the harbeth is more accurate. The eally tricky thing with spendor vs harbeth sound is if you compare it directly you may end up prefering the harbeth since in a direct comparison harbeth shows it has more dettails and Something more then the spendor. But when you have it both at home for few months the thing is different. The Spendor manage to get Inside your heart, the harbeth stay more in the brain. It is not a cerebral speaker but it is slihtly more cerebral then the Spendor. At the end of the ownership I had more memories connected with the big Spendor. Now I have a smaller one since I have a much smaller listening room (I have the SP1/2R2). In a way the Spendor give you a certain beauty that you are not going to appreciate in a comparative listening expecially against the Harbeth.
I say this since in a comparitive I always preferred the Harbeth but on the long run I always remembered the Spendor.
Both speakers a big and are difficult to positioning well expecially the Spendor.
Another good thing in favour of the Spendor: it is much easier to drive. The Harbeth is efficient but needs a strong ampli. It will sound with 40 watt but you feel something is not as it should. I have find out that from the big harbeth you can reach a very beautiful sound. With the Spendor you can reach many different beautiful sounds.