Polymer solutions are considered dilute when polymer chains in a solution do not interact with each other. One important step in the characterization of these systems is the measurement of their longest relaxation times lambda. For dilute polymer solutions in low-viscous solvents, this measurement can be very challenging through conventional techniques. Recently, several microfluidic platforms have been successfully employed to measure the rheological properties of weakly viscoelastic solutions. Nevertheless, a comparison between data generated from different microfluidic platforms has not yet been presented. In this work, we measure lambda of dilute polymer solutions for concentrations down to a few parts per million, by using two distinct microfluidic platforms with shear and extensional flow configurations. We consider three representative polymer classes: Neutral polymers in near-theta and good solvents, and a biological polyelectrolyte in a good solvent in the presence of salt. Relaxation times in shear flow lambda(shear) are measured through the mu-rheometer based on the viscoelastic alignment of particles in a straight microchannel. Relaxation times in extensional flow lambda(ext) are measured in a microfluidic optimized cross-slot configuration based on the onset of the flow-induced birefringence. A good agreement between experimental measurements from the two platforms is found. Experimental measures are also compared with available theories. (C) 2017 The Society of Rheology.