I love Rachmaninov, but he did very little pioneering. Scriabin experimented with harmony, tonality, and form to create amazing visionary works.
I love them both too. The Rachmaninov 2nd Concerto was the first piece of classical music that I became addicted to, I played the grooves off the first movement, maybe my parents were worried; then again I also latched onto the "Marriage of Figaro" Overture. I've always had manic mood swings though, this was just a manifestation perhaps?
Anyway I discovered Scriabin later as an ambitious teenage pianist; my teacher at the time had a box set of recordings of Welte-Mignon rolls, which contained Scriabin's roll of the Etude in d# minor op.8 #12. I immediately became obsessed, got the music and practiced it incessantly. My obsession soon included all of Scriabin's music I could get my hands on, recordings or sheet music it didn't matter. I'm still obsessed....
Rachmaninov is wonderful, I've played both of the Suites for two Pianos and the Symphonic Dances as well as several Preludes and Etudes Tableux and the Elegie op.2 #1. And the 4th Concerto and have studied the 2nd and 3rd Concerti and the Paganini Rhapsody. But Scriabin really got under my skin at an early age. My teacher (his fault he loaned me that record) at the time told me I would grow out of it, but I never did. I cannot fault Rachmaninov for being a musical conservative, innovation isn't everything, and Rachmaninov actually was innovative in his own quiet conservative way. It's interesting to contrast him with Scriabin, the wild eyed visionary who was going to save the world with his music. I think for many years Scriabin was dismissed as an eccentric madman by the stuffy classical music establishment, who BTW also had it in for Rachmaninov (read the Grove's Dictionary entry on Rachmaninov, it will make your blood boil

!). Fortunately Vladimir Horowitz and the 1960's made Scriabin "hip", a lot of his ideas anticipate the psychadelia (sp?) of that time. Rachmaninov was dismissed as schmaltz by the establishment but pianists played his works anyway (we know a good thing when we play it !

) And this became true of Scriabin in the 1960's and 1970's...
Scriabin is a little different, his works go through a logical but bizarre transformation and development and his late music is almost atonal, and when you put those works next to the early Chopinesque pieces, it's startling. I don't think people knew what to make of that for a long time. And if he hadn't died I think he would have been a significant force in 20th Century art and culture. It's easy to see him in 1920's France, continuing his harmonic innovations to the point where harmony could perhaps almost dissolve...I think as a thinker AND doer he was ahead of his time, and perhaps of ours as well. His early death was a tragedy for the 20th Century, who knows what he......oh well.
And Rachmaninov has been vindicated by the public , because we TOOK BACK OUR REPERTOIRE!!!!

from the the evil intelletual establishment!!

Anyway if I could only play one or the other, I would have to choose...Scriabin.
With no ill will or malice towards Rachmaninov....