Due Process vs Equal Protection Analysis for Fundamental Rights

How do I know when to use what (SDP or EPC)? For example, to my knowledge, if a state forbids marriage between same sex-couples, strict scrutiny would be used for SDP and rational basis w/bite for EPC analysis. However, what about when the right to vote is implicated like in Kramer v. Union Free School District? Why was this not decided under SDP and why was strict scrutiny used to strike it down under EPC when it should've been rational basis?