The main difference between GSM900 and GSM 1800 is the band because each one has different band as follows:
GSM900: 890MHz---->915MHz uplink 935MHz----->960MHz downlink
GSM1800: 1710MHz---->1785MHz uplink 1805MHz------>1880MHz downlink
So, each one has differnet band noting that GSM1800 has bandwidth that is three times more than GSM900(75MHz for GSM1800 while 25MHZ for GSM900)
Major difference is from Operator´s point of view because as you move from low frequency you can accommodate more and more users and can offer higher data rates as well..
Cell size is lower as compared to GSM at same transmit power.