Hello fans, I'm another Martin (Martin Nickison), but I love both
composers!  I will try to commentate on each.

Rachmaninoff (or Rachmininov) was a very emotional person; reflective
in his music.  He tended to paint emotional colors in his music, cast
over with a hint of grey (saddness).  His music is dense in harmonic
suggestions, although not usually stacked apon each other.  Rather, Rach
uses a lot of contrasts, glissandos (going from one note to another one by
quickly progressing by each key in between).

Prokofiev was much the 'happy genius' (for lack of a better way to explain
it).  His music is more of a difficulty of hearing and mastering for
preformance, whereas Rach is hard to learn, but popular consensis leads
towards me saying his music is easier to understand.  Prokofiev's music is
very chord-heavy, dynamic (loud), and harder to find the harmonic
foundations in when decomposing for a lecture.

Where to start?:

Rachmaninov: Piano Concertos 2, 3; Vocalise; NO SYMPHONIES (Very long);
Preludes (pick any book)

Prokofiev: Piano Concertos 3, 5, 2 (be warned....very loud); Love for
Three Oranges; Synthian (sp?) Suite; Piano Sonata 7

I am now ready to be put in my place by all of you who know more of these 2
than I do (most of you all):-).

Martin