People get the idea that the higher frequency systems (1800&1900MHz) outperform lower frequency systems (900&800GSM) because chances are they are physically located much closer to the higher frequency systems. Since the lower frequency systems propagate much better than the higher frequency systems, you are not required to have as many in your network to cover a given area. The only advantage I have personally seen in using 1800/1900 MHz GSM (or CDMA)in a network the disadvantage of poor propagation is negated by increased capacity (across a given area), increased flexability on network design (poor propagation requires more sites), and increased flexability on frequency planning (if your control channels only propagate so far….).