Older B&w 805 (N or S) vs new 705 S3

My following comments are purely a guess based on my experiences with B&W D2 and D3 speakers.

The older 805S will likely be much more refined than S3 in a lot of ways.  However, a couple of things to keep in mind.  The older generations used the woven kevlar midranges and midwoofers.  These had a nasty breakup problem in the upper midrange.  It sort of had its own unique texture, but it could also come across as somewhat of a distortion in the upper midrange or ringing.  Also the older tweeters were aluminum dome and have been described as sounding bright due to the metal dome breakup.

The newer S3 models have evolved with newer and better technologies to counter these old behaviors.  However, S3 is likely not going to be as refined as the 805S level and would sound "low fidelity" in comparison.

These had a nasty breakup problem in the upper midrange. It sort of had its own unique texture, but it could also come across as somewhat of a distortion in the upper midrange or ringing.  

You've measured it have you? Can I see the results?

I have a set of 705 S2’s and a set of 805S and I’m powering them with a McIntosh MA352, and I have found a couple of differences between the two. 
The current 705 S2 are very well made and the fit and finish are beautiful. Sound wise I found them to be accurate and very detailed, almost to the point they could be a bit fatiguing at higher levels for long listening sessions. I normally do not play my music to that point, but have noticed it. The bass response is pretty good for a stand mount speaker. If you are looking for deep bass, you will need a sub. I was running my 705’s with a B&W 600 sub and it certainly added a bit more bass. 
I ended up running across a perfect set of used 805S’s at my local audio shop that had the original boxes and papers. The 805’s are wonderful. I have found that the highs are very smooth and detailed yet not over powering or bright and not fatiguing at all. The 805’s seem to have a bit more lower end and they sound really balanced and the bass response is very good, but again if your looking for deep bass, you will need a sub. The fit and finish are really nice and while I wish they were piano black like my 705’s, the cherry are beautiful as well. For the money, I would go for the older 805’s over the 705’s. Especially since the 805’s are the entry into Bowers higher end speakers. 
I hope my comparison helps in making your decision, especially since I ran into this post a few weeks after you posted it. Best wishes and enjoy your music. 

This is a genuinely helpful group. Thx to all.