GPRS offers data speeds of 115 kbps, and can theoretically reach 160 kbps. While at EDGE data speeds sbesar 384 kbps, and in theory can achieve 473.6 kilobits per second. EDGE speeds generally three times higher than GPRS. This is possible because the techniques used in EDGE modulation (EDGE uses 8PSK, GPRS uses GMSK) and different methods of fault tolerance with GPRS, and also an improved link adaptation mechanisms. EDGE also uses a different coding scheme with GPRS. In nine kinds EDGE coding scheme, whereas in only four GPRS coding scheme.
Source : http://id.wikipedia.org/wiki/Enhanced_Data_Rates_for_GSM_Evolution#Kapasitas_EDGE_Sebagai_Teknologi_Data_Transfer_Tingkat_Advance