If I've interpreted this correctly
http://www.ukmi.nhs.u...docs/escitalopram.pdf
there's not a great deal of difference. Quote: "There is no
good evidence to suggest that escitalopram offers any therapeutic
advantages over citalopram."
That document appears to show that escitalopram is claimed to be an advanced preparation of essentially the same drug which (it is claimed) makes it faster working and more effective. However (another quote):
"The evidence to support the claim that escitalopram has improved efficacy and a faster onset of action than citalopram in the treatment of depression is not compelling."
So it seems that the manufacturers are offering escitalopram on the basis that it's the "new and improved" version of citalopram (which, unsurprisingly, costs more!) but that medical opinion is, as yet, unconvinced by the claims.
Chris