I went from Harbeth 30.2 directly to the M3 Sapphires this March. I have been using the M3S 2-4 hours a day since March, have been home a lot this year, and had the Harbeths for about 15-16 months. They are very different speakers with vastly different presentations. The Harbeths are fantastic speakers. They are obviously based on the BBC monitor as well as many other speakers. They sound detailed, smooth, warm, pretty punchy for it's size, and IMO stunning with the Silver Eucalyptus veneer. I listen to mostly vocal and acoustic music and they are perfect for that. Before I bought the Harbeths I read a lot of reviews and comments about them and many people describe the midrange as sounding right, after owning them for a while I cannot describe it any better, I would just add natural to it. Compared to BBC monitor designs other speakers will sound Hifi-ish in the midrange.
So why did I get rid of them? They are incredible for vocal and acoustic tracks but fall flat with anything dynamic or complex. Occasionally I would like to listen to large orchestra pieces or big band, and they just can't do that well. I found that during the 15-16 months I had them I had almost completely avoided those genres. They also need a sub, I have a Rythmik F12 which paired nicely with them. They are also relatively inefficient, spec'd at 85db but tested by Sterophile at 83db. Alan Shaw the designer always recommended at least 100 watts of solid state power. I used a Benchmark AHB2 and a Jeff Rowland 535 during my time with them and both sounded great.
While looking to move on from the Harbeths, I found a pair of Magnepan LRS'. I like them so much I though my search was over and I went to demo the larger models. The one I wanted was the 3.7i but they were just too big. I also have a really bad room mode at 50hz. I have a square room at 14' x 14' x 8' which is horrible for bass. The Maggies introduced me to open baffle and how it can help with a room like mine. That led me to finding Spatial from all of the glowing reviews from last year's audio shows. I emailed Clayton to inquire about M3 or X5 and he called me back that evening. I ordered the M3S within 10 minutes of hanging up with him.
The M3S sounded lean at first coming from the Harbeths, I had just come from the full bodied, seductive, lush, rich midrange of the Harbeths to the these speakers that are not even broken in. As time went on and they continued to break in I find them to be the most neutral speakers I have ever owned. I still feel vocals sound bests on BBC designs but M3S are no slouch. They have incredibly fast and deep bass that doesn't excite my room mode. If you owned Spatials or plan to demo them use these to tracks to test bass, The Robots, Kraftwerk and Dacoit Duel, AR Rahman. I don't use my Rythmik with these since I don't need to go any deeper and I don't think it can keep up. You can also use any amp with these speakers. I have used 10 wpc tube amps with these as well as the 100 wpc Benchmark. I also tried First Watt F7 and SIT3 and IMO these are the best amps for Spatials. I have settled with the SIT although I have a Pass Labs 30.8 in the house since yesterday for a shootout.
In short, they are very different designs and presentations. If you value mids over all else and want more flexibility in placement Harbeths are great, if you have a troubled room like mine, can have the speakers 3' into the room or more, value neutrality and quick deep bass Spatials are one of the few brands to consider.