The difference will be in the price of the rad: the second place sound as if they are charging for one in a Citroen bag, the first place are charging for a non 'genuine' replacement........but don't worry, it'll be from the same factory as the 'genuine' rad.....
Eurocarparts charge between �100 and �130 plus vat for an XM rad, depending on which XM it is, so the first garage sounds like its quoting a fair price based on getting the rad in at trade from Eurocarparts or GSF or similar........as indeed does the second one if they are using a 'genuine' Citroen radiator.