I bought a P6 (not a Rega Planar 6) a couple of years ago , but before I doing so, I thought I’d make some interesting comparisons in the the listening room at the HiFi shop and share my thoughts with you. Whilst I believe there is nothing wrong with my hearing I always take one of my sons (in his 20’s) to give an honest opinion (they have their uses you know!) 
All my listening appraisals used the same amp and speakers (Naim SN3+ Spendor A3):
First up: Rega P3 (with Exact Cartridge): Nice open sound with good bass. This is a great turntable which not only sounds good but also looks the part!..no wonder it is so popular!
We then tried the P6 with the same Exact cartridge: The sound characteristics (to us at any rate) sounded the same. Possibly a little bit better …but not by much!
Next up we tried the P6 with the Ania cartridge. This gave more improvement - more control and definition with a warmth that drew you in. I would say that this change was more pronounced, I think the external power supply and Fono MC really contribute to the uplift here.
Curiosity got the better of me at that point. We just had to try the P8 with the Ania cartridge. After a few minutes of playing a couple of songs we thought there was very little difference between the P6 and the P8 using the Ania (and Fono MC). Both excellent in our opinion but nothing to call in overall SQ . I realise that this statement might incite lots of controversy on this forum, but I’m just reporting my own subjective opinion!
At this point I really wanted to test the P8 with the Alpheta cartridge and then test it back on the P6, but there wasn’t one available to try.
As a quick break from Rega turntables I then turned my attention to a Clearaudio Concept turntable. This was a nice turntable and you are almost reassured by the weight. The sound was very good also, and comparable to the P6 Ania combination. But I wasn’t a fan of the magnetic bearing on the pivot and whilst it worked perfectly I had reservations about it’s reliability in years to come.
In the end I came away with the P6 with Ania (and Fono MC stage). For some, the P8 may be worth the extra but the difference to me using the same cartridge was too close to call. I also was not a fan of the exposed cross section of the skeletal plinth . I liked the shape, but it looked unfinished to me with that exposed foam edge. I didn’t like the minimalist dust cover very much either!
Over 2 years in now, my P6 has performed flawlessly. Not a hint of any sag in the plinth and the rotation speed is very accurate ( I do check these things!). Having a full sized dust cover that completely covers the deck is a big plus for me. In a few years, I’ll probably upgrade to the Ania Pro (wasn’t available when I bought mine) or might even try an Alpheta with an Aria phono stage with an upgraded tonearm, but until then I am very pleased with the P6/Ania combination (you get the Ania. at a reduced price if you buy ready fitted to the turntable btw) Whilst a a good turntable is essential for great sound, I believe that the cartridge / phono stage probably gives the biggest contribution to overall SQ