Aqueous mixtures of polyelectrolytes and oppositely charged surfactants form clear and homogeneous solutions if either the polyelectrolyte or the surfactant is present in excess. Around charge equilibrium, macroscopic precipitates are formed. Near the phase boundary on the polyelectrolyte-rich side, nanometric polyelectrolyte/surfactant complexes can greatly increase the viscosity of aqueous solutions. This behavior is governed by the composition and chemical nature of the polyelectrolyte and surfactant. Here, we investigate complexes consisting of the polycation JR 400 and two different surfactants, namely, sodium octyl sulfate (SOS) and sodium tetradecyl sulfate (STS), which only differ in the length of their alkyl tail. Using small-angle neutron scattering and neutron spin-echo spectroscopy, we find that STS forms mixed aggregates with JR 400 which results in a pronounced increase in viscosity. Such an increase is not observed for SOS where no mixed aggregates are formed. Comparison with atomistic molecular dynamics simulations shows good qualitative agreement.