Pierce was maybe a hairline above Ray. Head and shoulders above him, no. When they had paired up, they had very similar career numbers, and had limited postseason experience, with each having some signature games (Ray's 46 vs. Sacramento/43 vs. Philly, Pierce's triple-double/40 vs. Indy, etc.). Both had been in the conference finals once. Ray played very well in defeat (shot 48 percent, 48 percent on threes, 2.5:1 assist to turnover ratio, went gonzo in an elimination game with 43 to tie series 3-3), and Pierce frankly soiled himself (shot 36 percent, 21 percent on threes, 1:1 A:TO ratio).
In Game 7 against the Lakers, Ray (3-14) had to guard Kobe (6-24), and Kobe didn't have to guard him. Kobe got to sit back and play 10 feet off Rondo to rest for offense. They basically played to a standstill of coldness, while Pierce (who was 5-15 himself) lost his battle with Metta World Peace (20 points, dagger three) at the three spot. I know Celtics fans like to blame Ray for Game 7, and they certainly could have used another shot or two from him, but damn, Pierce needed to win that head to head battle with MWP and his psychiatrist and he lost it.
(on edit, the reverse was true in that first Cleveland series in 2008, when Pierce was dealing with LeBron, and Ray was losing a battle with Szczerbiak - I'm actually very thankful to Pierce, and PJ Brown, for carrying Ray through that series and saving his rep. Pierce's performance in Game 7 there could be his masterpiece).
Now, in the Big Three Era, I freely agree that Pierce was - part by design of their system, and part skill level at that stage of their career - head and shoulders better. But when you start talking HOF - Ray had one advantage over Pierce, even back in 2007 (moot now, since they will both go). Ray was the best ever at something (or in the discussion among the top two or three). Pierce doesn't have that signature thing that would have made people remember him (which is different now that he was the alpha dog on a great Celtics team that people will remember, with a Finals MVP).