I'm not a huge fan of Schirmer. I have the Dover edition, which also has the etudes-tableaux included. Boosey & Hawkes is worth a look too, but I'm not entirely sure it's any better than Dover. Urtext is not as critical for Rachmaninov as it is for Beethoven, Bach, Mozart, etc. I think there is a barenreiter edition, but I don't think it's necessarily essential.