Marginal utility ratio 、marginal rate of substitution

The main difference is that marginal utility (MU) ratio has been specified the object as utility but for marginal rate of substitution (MRS) which the object is not specified.

For example;

MU from one more unit of good 1 starts declining after total utility is reached (maximum).
Further, the MR ratio is comparing the MU derived from 2 goods.

But for MRS, which the object can be labour vs capital, present vs future consumption, food vs machinery ( production froniter), which are all expressed in units.