[font color=\'#000000\']I would consider the MKII over the 1Ds for two reasons -- better High ISO and most importantly, battery life. When I backpack, weight is critical and with DSLR's the battery is what weighs... I have not done a true study, but it appears my MKII gets at least twice as many captures per battery charge than my 1Ds.
However, the 300d would not be out of the running either -- decent high ISO, decent battery life and even lighter in weight.[/font]