Harbeth vs Vandersteen vs Sonner

One thing I can say about Harbeth in comparison to others (not necessarily the other speakers you are considering), is that when comparing them, other speakers can sound more "exciting", "involving", or any number of similar adjectives used to describe what some will define as "better."
The goal with the Harbeth sound is accuracy and neutrality.  They are designed to not add anything to the sound that was not part of the original recording.  For listeners of rock, this can be less exciting than other speakers that accentuate different frequencies to add body, warmth or excitement.  
For listeners of jazz, acoustic based music or vocals, there is not much better than Harbeth.  
I do not advocate that you buy Harbeth, even though they are the best speakers I have owned (I've owned several pairs).  BTW, I also own a pair of Sonus Fabers that I find to be fantastic.  While they share some things with Harbeth, I would not say the SF speakers sound anything like the Harbeths, and definitely not as good IMHO.  Still, I enjoy them for the simple reason of them sounding good in the room I am using them.  
The one thing I will say in favor of Harbeth is that they are very popular, and can be somewhat hard to find right now.  This is not a bad thing, in that they are easy to sell if you change your mind later, and they tend to hold their value better than many other brands.  
Whatever you decide, please make sure to share your findings here.  
Happy listening!
Peter

@denon1 My Sonus Fabers are the Olympica Nova I model.  I prefer my Harbeth 40.2A's over them but keep in mind the Harbeths cost 3X the price.  My 40.2's are in a room that's about 12' x 14', which is "supposed" to be too small of a space for the speakers, but I find the sound to be perfect for the space after tuning and tweaking.  
For your room size it really depends on your budget.  The 40 range of Harbeths will not be too much for your room, if not too much for your wallet.  If you can't go that high on your spend you should consider SHL5+ or 30.2's.  I used to own a pair of 30's and loved them.  I have never owned SHL5's but I have heard them in a store demo.  They sounded great.  
@audition__audio I have heard a few others say what you said about the lossy cabinet design of Harbeth, but in my experience you are in the minority.  I will say that there is no one speaker that will suit everyone's tastes (duh!), and this is a good thing.  I don't know your specific experience with Harbeth but it sounds like you don't like them.  I have found that they sound their best after proper match with amplifier, source components and cabling.  I think this is true for every speaker (probably), so it's no revelation what I'm saying, but unless you have owned a pair of Harbeths and have truly worked at getting the most from them I'd say you really haven't heard them in their best light. 
On the other hand, if you have experienced them fully and simply didn't like the sound, that's OK.  I just don't agree with the assessment that lossy cabinets are at fault.  The design is intentional, and right, and clearly this opinion is supported my legions of Harbeth loyalists.  I personally think B&W speakers sound very unimpressive, but clearly my opinion is wrong according to legions of loyal B&W fans, so I figure I haven't ever heard them in a proper setting.  The fun part about this hobby is comparing experiences that a wide and diverse group is having.  
Cheers,
Peter