Presumably you will be seeing it at the Royal Festival Hall if you are in London.
I don't think you will be disappointed with the third concerto which, to my mind, is even better than the second. Rachmaninoff had one of the greatest musical minds of any century and it takes, not only a musical sensibility but also a prodigious technique to play him. Who is the soloist on June 6th?
Tchaikovsky's 6th is another great work in the Russian repertoire but it ends in a somewhat a negative way. Strangely, it's the one that conductors seem to prefer as it's the most frequently performed but I think that the last movement is too much of a change after this very typical Bernstein performance with the NYSO, at which the audience applauded before the last movement was played.
http://youtu.be/EbEGSbipAA8