Last Speakers Aerial 20T or Vandersteen 5A

I'm sold on time aligned and phase coherent speakers. With that said, I'm biased for the Vandersteen. It also allows for fine tuning of the bass (as Sean points out above) and it's much less money for just as well built but more accurate speaker.
The ribbon tweeter and dual ported woofers would scare me away immediately. I have never heard ribbons and dynamic drivers ever blend correctly to the point where I didn't think you had timing issues.
You may not prefer the Vandersteen, which is of course fine. A lot of Audiophiles do not get excited over truly accurate designs. The are use to hearing colorations and the tweeters sound arriving way before the rest of the speaker giving that "Illusion" of transparency.

Why only two choices? For this kind of money there are other fine performers. Your amps are not very common, and few here will know how any speakers will exactly sound with them. Would you be willing to replace them if you really like the new speakers? For example, Kharma speakers sound incredible with LAMM electronics. Or, Michael Green Audio Classic Chameleons with outboard crossovers have a unique design and sound signature. Also, the cost issue. Does it make any difference for you to spend $25000 instead of $15000 provided you get a much better sound? Vandersteen is a popular choice, not everyone's though. Do not get affected by the herd instinct. Trust your ears. You have one of the best front ends available, both analog and digital; it is going to be difficult for speakers to match it.

Inna: You make a good point. There are indeed lots of other great choices; however, the post specifically asked about the Aerial vs. the Vandy. And I'd hardly classify Vandersteen buyers as caving to "herd instinct" -- or mentality. In fact, many audiophiles, myself included, for many years dismiss Vandersteen as being too common -- until we've owned much more "exotic" speakers and discover that the Vandersteens actually deliver great sound for not that much money, relatively speaking.

Are there better speakers? Sure. But probably not for the money. IMHO.