Vandersteen

@bdunne,
I assume you are considering buying used. So, cost differential would be minimal.
In any case, the biggest difference between the two models would be that the 3a sigs would have greater bass capabilities.
If you love Vandies like I do, then the next best upgrade would be to add a pair of 2w(q) subs which will bring you almost equal to a pair of 3a sigs.
(I upgraded from the 3a sigs to a pair of Treo's and use a pair of 2w subs. Mr. V. says it gets me to 90% or a pair of Quatro's-These are non-CT versions). So, if you want to upgrade in the future, you can still keep the subs.-Which, by the way, are the best subs I have ever used, despite not having a 'q' adjustment.
One of the things I didn't like about the 3a sigs was the footprint. The Sound Anchor stuck out like a sore thumb and made positioning much more difficult that the 2's and Hence, my move to the Treo's.
You should PM John Rutan at audioconnection (his store and Agon ID). He can give you the best information. And, he's a great guy, too.
Bob

@bdunne 

I agree with cedargrove, moving up the line to a pair of a pair of Treo's or, better yet, a pair of Quatro's, would be the way to go.
Regarding the move to 3a sig's, I would add another 2wq sub-which would get you into 90% of a pair of 3a sig's. 
As cedargrove mentioned, upgrading to an Ayre amp/integrated or an Atma-sphere amp will really open up those 2ce's.
Bob