The nonlinear dielectric properties of 0.9Pb[Mg1/3,Nb 2/3]O3·0.1PbTiO3 (PMN-PT) and Ba[Ti 0.85,Sn0.15]O3 (BTS) paraelectrics were studied experimentally and theoretically. The nonlinear dielectric response was measured in the parallel plate capacitor configuration, whereby the low frequency linear permittivity (ε33). and the higher order permittivities (ε3333, ε333333) at 298 K were obtained as å33PMN-PT=2.1×10-7 and ε33BTS = 4.1×10-8 (F/m), ε3333PMN-PT = -4.9×10-20 and ε333333BTS = 9.85×10-34 (F 3m/C2), and ε333333PMN-PT = 7.6×10-33 and ε3333BTS = -7.3×10-21 (F5m3/C4). By using a self-consistent thermodynamic theory in conjunction with the experimental data, we compute the E3 dependence of electrostatic energy AG, the field-induced polarization response P3, the thermodynamic tunability ∂2P3/∂E32, and prove that electrostatic free energy has to be expanded at least up to the sixth order in electric field define the critical field |E*3| at which maximum tunability is attained. We also show that |E*3| is a function on |ε3333|/ε333333 only. Consequently, we find |E*3|PMN-PT = 8.6×105 V/m and |E*3|BTS = 8.0×105 V/m. We compute the engineering tunabilities as ΓPMN-PT = 65% and ΓBTS = 55%, and then define a normalized tunability ξ to take into account the |E*3| parameter. Thereof, we determine |ξ|PMT-PT = 8.1×10-5 and |ξ|BTS = 6.4×10-5 %/Vm-1. Our results reveal that |E*3|BTS > |E*3|PMNPT although ΓBTS < ΓPMNPT, unequivocally showing the need for defining a critical field parameter in evaluating the nonlinear dielectric response and tunability in particular, and in nonlinear dielectrics in general. The results also indicate that the nonlinear dielectric properties of PMN-PT are an order of magnitude higher than that of BTS, which we discuss in the context of structure-property relations of relaxors.