It depends on personal choice. Note that solid grab handles will block the complete extension of sun visor towards the windshield. Whereas the fiber or nylon grab handles (roll bar) can be folded to allow visor extension.
Both offer almost equal safety for everyone in the vehicle. The solid bars are slightly superior because they are metallic built, but they may rust and corrode. Some metal grab handles will cause uneven cranks in the vehicle because of uneven expansion during summer and winter seasons.
Nylon grip grab handles are all-weather resistant. Though they do not offer similar functionality as metal handles, they are easy to install and can fit on to most vehicles.