This paper is devoted to study and compare dynamics of primary Linear Oscillator (LO) coupled to cubic and Vibro-Impact (VI) Nonlinear Energy Sink (NES) under transient and periodic forcing. The classic analytical procedure combining the approach of invariant manifold and multiple scales is extended from the analysis of steadystate resonance to other regimes, especially Strongly Modulated Response (SMR). A general equation governing the variation of motion along the Slow Invariant Manifold (SIM) is obtained. Numerical results show its convenience to explain the transition from steady-state response to SMR and the characteristics of SMR for periodic forcing. Targeted energy transfer under transient forcing can also be well understood. Experimental results from LO coupled to VI NES under periodic forcing confirm the existence of SMR and its properties (e.g. chaotic). They also verify the feasibility of the general equation to explain complicated case like SMR in experiments.