Acoustic Signature Vs VPI Prime Signature

"I have been told AS and Clearaudio are best option for serious music listening"

Run away as fast as you can, when anyone mentions "the best" in audio.

I have an Acoustic Signature Wow XL and before that had a VPI Classic 2 with a Classic 3 tonearm.  I have also owned the RPM 9.1 and a couple of Clearaudio tables (Concept and Emotion).  So not the higher end tables you're considering, but here are some comments.

Pro-ject makes some decent turntables.  Set up properly with a good cartridge, they can sound really good.  I've heard the Xtension 10 in a friend's system and the owner of a local(ish) hifi shop had one as his referrence for a while.

My AS and VPI tables were a significant step up in build quality from the 9.1 and both had better sound quality.  The two models I had were more alike than different in terms of sound quality, with the AS having a little more detail and drive than the VPI.  The VPI was built a little better and I really liked the ability to use the VPI Periphery Ring which was a big benefit in general and especially for slightly warped records.  

As mentioned, there's no "best", only what works best for you and your system.  Part of the reason I chose the VPI and AS tables was their relatively high mass.  I have some placement limitations and it was helpful that both were really good at rejecting acoustic feedback and dealing with a bouncy floor.  Your cartridge, sonic preferences, phono stage, aesthetics, etc. all figure into the equation.

I have the AS Triple X with the 12" TA-1000 Tonearm. 
For me this is my forever table. The high mass is perfect for my oak pine floors with no rumble or skipping. I can jump up and down and the table is solid. Unbelievable build quality and beautiful as is the Double X. Sonically this responded unbelievably well with my initial Hana SL then ART9xa and finally now the Umami Red. Agree with biggreg that it's just one part of the equation but I think either way you go will be a significant step up. I can say that I have had awesome customer support when trying to get info on setup or questions about my Triple X. Good Luck and let us know what you settle on.

I have been a long term owner of a VPI Aries turntable. I loved the turntable. I really trust VPI to deliver outstanding After 20 years with a VPI turntable I switched to a sprung table... Linn LP 12. It is an outstanding table... but with the livelyness of a sprung table. I added a SRA isolation platform ($3,400) and it added a lot of the solidity and slam of the VPI. 
 

I would unhesitatingly purchase the VPI. Great products, been in business for a long time. I think you will be very impressed. 

Many thanks folks for wonderful feedbacks.

One area interested me in AS is their tonearms, more optional and far expensive than VPI top of the line fatboy, i assume with their top tonearms can deliver higher performance.

If the AS arm is what you like check out the Pure Fidelity Harmony which can be ordered thus equipped. I've got about two months on mine and recommend it very highly. Feel free to PM if you want to know more.

The new Acoustic Signature (AS) Neo turntables carry a 15 year warranty which is what pushed over the edge to get one (Hurricane). After much deliberation, I opted for the Kuzma 4Point9 tonearm instead of the AS-2000. The next level up from the AS-2000 is way too expensive. I wish that I could tell you how it sounds but I am waiting for delivery of a Hana Umami Red phono cartridge. Then I have get the cartridge set up, so this will take a while longer.

If you go AS, which I would for many reasons, do not get their tonearms. I bought a Typhoon for my home in the US, I liked it so much that when I came back to Spain, I went and listen to the Invictus Junior and Ascona. My partner is getting a Junior for  Xmas, with a Kuzma 4point and SAT CF1-9Ti tonearms.

The VTA and azimuth in the AS tonearms are rather rudimentary, kind of like antiskating in VPI, which I have and I am trying to sell becasue after 7 months I realized that VPI is not for my taste. 

PM if you want more info in my experience with AS in the USA, the dealer I bought it is great to work with.