5D MKII or MKIII

I have a Canon 5DMKII which has developed a bad case of sensor rot (water spots on the sensor or the filter in front of the sensor).  I am thinking that I need a backup camera in case it is not practical or too expenseive to fix this problem.  My question is whether I should get a 5DMKII for $1800 or a 5DMKIII for $3500?  I shoot almost exclusively landscape photographs, so lots of the upgrades in the 5DMKIII don't seem relevant to me.  Any suggestions would be appreciated.