That is a good question. I assume the DCS1800 cells are better because :
1/ the 1800 frequency band is much cleaner (more channels available –> frequency planning result in better C/I)
2/ the 900 cells are congested.
3/ the 1800 provide less coverage (-10dB compared to a 900) ==> use the 1800 to carry as much traffic as possible, and leave 900 capacity for faraway or indoor traffic, or for EGPRS. Indeed EGPRS enjoy high RXLEV a lot. (MCS9 needs > -85dBm or so)
Now in some networks, that might not be valid… but I think it works in 99% of the case 🙂