If you plan to push EGPRS on the DCS1800, then you should use the highest MCS available. The MCS is adapted dynamically. If the radio is good, the best MCS is used.
For me, the best strategy is to push the EGPRS on the GSM900, and the voice on the DCS1800. Thanks to speciic parameters (NC2 reselection and redirection) you can achieve this kind of strategy.