firstly look at the spec sheets on www.motor-v.co.uk this has the main differences in wieght length gear ratios etc. then go on www.ralliartuk.co.uk which is where they sell them and it should tell you the differences in air con, bhp etc.
About 6K if bought from Ralliart UK
GSR has ABS and AYC, RSX does not
RSX has no warranty, GSR has 3 year warranty if bought from Ralliart
RSX has quicker steering rack
RSX probably the quicker straight line car due to lack of ABS and AYC to weigh it down
RSX only available in White.
Was over at Ralliart last weekend. Apparently all Silver RSX's are sold but they have a lot of white ones in stock! Heard that there were only ever a few silver ones and quite a few of those got exported, so if you get offered a silver RSX, count yourself lucky.