Funny you should mention the P3esr, that was number one on my list two years ago, when I ran into Gene's ad for the Spendor. Given they were both in the same price range & both had their origins as BBC monitors I thought how different could they be?
Granted the Spendor is sealed-box while the Harbeth is rear-ported, I think I have enough space between speaker & wall that rear-porting shouldn't be too big an issue. Both have received accolades from the hi fi press (though I think the Harbeth has made a few more "best of" lists than the Spendor). I just don't think I could pull the trigger on the P3esr without being positive that it would constitute an upgrade. My Spendors by all accounts are different from any of the company's previous models, so you can't really go by past experience.
As luck may have it the closest Proac vendor to my location in NYC also carries Harbeth, so if I can figure out how to get there w/o a car I might have the chance to compare them.
Ironically for a budding audiophile I'm a bit hard of hearing (not from music, though;-), so I generally rely on cost & reputation to make my purchasing decisions. The differences between this & that component aren't obvious to me until many, many hours of listening, so auditioning speakers at the store is something I'll do, but it's not the last word in what I decide.
I just read up on the Aerial 5 ... nice little speaker, btw.
Oh, & the funny thing, I had written to Gene about buying an Exposure 3010 myself this past December, to upgrade from my Rotel amp/Luxman pre combo. He wrote back recommending the LFD quite strongly. I knew he wasn't just out to make a buck 'cause he'd helped me out for free when I blew a woofer on the Spendor a few months earlier. So I dug a little deeper, spent more than I ought at the time, & sprang for the LFD.
Very glad I did.
I got the other items on special--e.g. the Sony was a floor model at J&R, so that helped. But the problem was I made so many upgrades at once that I couldn't possibly tell to what degree each item is responsible for the terrific sound I'm now enjoying.
That said, I think I would have liked the Exposure, too:-)