Substitution is easiest to use when one of the equations is already solved for one of the variables. This makes it easy to substitute the solved equation into the other equation and solve for the remaining variable.
Elimination is easiest to use when the equations are both linear and have the same coefficients for the same variables. This allows you to add or subtract the equations to eliminate one of the variables, leaving you with a single equation that can be solved for the remaining variable.
