We present a comparative neutron spin-echo and dynamic light scattering study of the chain dynamics of the dynamically very flexible poly(dimethylsiloxane) (PDMS) with the orientationally hindered polyisobutylene (PIB). Both polymers exhibit the same static rigidity. In the melt PDMS follows the Rouse dynamics up to momentum transfers of Q = 0.4 Angstrom (-1), while PIB displays a strong influence of local dynamics already above Q = 0.15 Angstrom (-1). In dilute solution the dynamic structure factors and the diffusion coefficients of both polymers were studied over a wide temperature and & range. A comparative evaluation of the PIE intrachain dynamics on the basis of PDMS results, which are taken to reflect as in the melt "ideal" relaxations, shows that intrachain viscosity effects are the leading mechanism displayed by the Rouse and Zimm models. The characteristic relaxation time of the intrachain viscosity to agrees well with the rotational barriers in PIB, corroborating the underlying physical idea of a delayed redistribution of conformational states to be at the origin of internal viscosity effects.