Choice of Vandersteen models

The first version of the wood Quatro has a lower midrange suckout. Kind of like some older DM B&Ws but worse. 

the 5a is no longer in production, so we are talking used
they are about tge same money 5a and Q-Wood used
the mid suckout was floor bounce and was solved in collaboration with the reviewer who noted it ( and gave a glowing review ) later models have a tweak to make the room interaction less critical, but you could call factory.
As for value used later...they are pretty much tied today, easy form factor and WAF with the Q, better ( much ) lower bass in the 5a assuming you are in a room that allows for that..
as to uogrades, available to origional purchasers only, and priced that way.
long time 5a and now 7 owner with a pair of Treo coming...

speak to John Rutan at Audioconnection. 5s need HUGE room college auditorium size. 

Midrange suckout? Are you referring to Stereophile's measurements? That "problem" is because Atkinson has to measure at 50" in his "quasi-anechoic" technique, and he doesn't always use the manufacturer specified height. This severely handicaps first-order designs in particular. Compare, for example, Stereophile and Soundstage's measurements of the Thiel CS2.4. Stereophile shows a big "suckout" in the upper midrange/lower treble while Soundstage shows near ruler flat response from ~35-20K cycles. Why the difference? Soundstage has a real anechoic chamber and, more importantly, measures at 2 m = 79".

I disagree that the 5’s need a HUGE room. I have a fairly small room and use 5’s and they sound quite good.  A bigger room would be better, but it’s not a necessity.  Having said that, the Quatro Wood may be a better choice for a small room.  I got my 5’s here for $5k several years ago, at the time Quatro Woods were fairly new so very few used for sale and when they came up they were in the $7k range.  The bigger issue is finding a used pair, don’t see them too often so if you are picky about which wood species you want then you may have to have a lot of patience.  Good luck!