Nextel-Sprint will be going from iDEN (current Nextel--offshoot of CDMA) and Sprint (current CDMA) to CDMA 2000 if I remember right.
Definitely not GSM.
And durability, yeah, go Nextel. Specifically for your dad, find him an old i700 plus on ebay. Those suckers are invincible. I have personally dropped one from about 25 feet up onto pavement, and it worked fine. Along with dragging through gravel, bouncing off of rooftops onto the ground, toted around in rain/sleet/snow, splashing beer/pop/gatorade/food onto...
The old i700's are a good weapon too. I think you could hurt someone with them. Looks like you can find 'em pretty cheap on ebay right now.
Otherwise as far as GSM/most-newer-phones and durability goes....I dunno. Those are all flashy and camera/video/pda/videogame and oh yeah a phone all tucked into the smallest cases now. Even my Nextel i60 which I thought was sissy after the i700+ is head and shoulders over my v265 (Motorola/Verizon/CDMA). Even an i60 or i90/95c would be sturdier than most newer phones. I dunno about many of the new Nextels though.