It's not the network. It's the hardware.
Qualcomm (inventor of CDMA) more or less forced RIM to do all the CDMA processing in software rather than hardware. So as a result, the 7750 is really slow, and it's the DogBerry.
The 7250 uses a brand new chipset, that does CDMA processing in the hardware, so the 7250 is much faster: http://www.blackberryforums.com/viewtopic.php?t=3009
Also, GSM/GPRS BlackBerry generally have much better battery life and RIM put more features in the BlackBerryOS of GSM/GPRS -- you no longer need BES/MDS to run TCP/IP Internet socket applications on GSM/GPRS networks. This isn't the case for CDMA networks (yet, at this time).
Also, it appears that Verizon (at this time) does not support BlackBerry as a modem for laptop on the 7250 (yet.)