Both seem logical, but unfortunately it´s not that easy.
First: La Pialade is the 3rd (we can even say 5th) wine from the whole Rayas/Fonsalette-estate.
Everything that isn´t used in the Fonsalette and F.Syrah (and Rayas/Pignan resp.) ends up in La Pialade - so it´s usually only 50-60% Grenache, with the rest Cinsault and Syrah, possibly even with tiny parts of Grenache blanc, Clairette and Marsanne, grown some 30 km north of Rayas on completely different soils. I very much doubt that a single drop of Rayas ever is in La Pialade, although it might be possible in very weak vintages …
(e.g. in 1982 all of Rayas/Pignan went into Fonsalette …)
But the hyrarchie usually is:
Rayas is (when necessary) declassified into Pignan (never the other way round) … some lesser Pignan might go into Fonsalette (although very rarely) - and weak Fonsalette goes into La Pialade.
Chateau des Tours is an independent estate - and has enough cuvees itself to cope with lesser material.
There is indeed a strong resemblance between des Tours and Rayas, but it´s got only little to do with the soil (which is different), much more with the wine-making and philosophy. The Vacqueyras is close to 100% Grenache, but the Cotes-du-Rhone (G. + Cinsault, Syrah, Counoise) and the VdP (+ Merlot !) are not.
Although I´m visiting and tasting at Rayas for almost 25 years I´m always puzzled and still remain igorant how exactly Jacques did it - and Emmanuel now does it. The cellar looks dirty in 19th century fashion like ever, the anchient casks (!) are certainly very important … and another major factor are the very low yields. Moreover ripening happens very slowly (no stones in the vineyards) with great patience, no hurry, late harvest (risky!) with close as perfect ripeness as possible … with very traditional vinification (no destemming) and not extremely long maceration.
(they do NOT have a consultant or oenologue !!! )
Yes, many things several other producers are also practicing partially or predominantly - but still Rayas and the other cuvees are simply UNIQUE … there is no substitute for the real thing, Rayas is not only a wine, it´s a unique strong character with immense individuality …
Chateau des Tours is a good introduction - and Pignan is quite close to Rayas in some vintages (in others its far less interesting, sweet and intense …), but who hasn´t tasted a fine MATURE Rayas has definitely missed something important, IMHO more than never having a Romanee-Conti, La Tache or La Romanee.