Proliferation index is the average number of divisions that all cells have undergone after they had been stained by a cell proliferation dye (e.g. CFDA-SE). Mitotic index is a percentage of total cell population that stain positive for phospho-Ser10/Histone H3 (by other words, the percentage of mitotic cells). Thus, these two indices quantify two very different things. Mitotic index of populations composed of cells arrested in M phase will be high, but the mitotic index of populations of cells arrested in G2 phase will be low (unless arrested in very late G2 phase). On the other hand, proliferation index will indicate how many cell divisions were completed on average per cell (before the cells were arrested in G2 or M-phase of cell cycle, if they were found arrested in G2 or M). The answer therefore depends on specific scenario: if the cells were stained by a cell proliferation dye and then treated with some agent that induced G2 or M arrest, then high mitotic index will support M-arrest as opposed to G2 arrest and the high proliferation index at the same time will imply many cell divisions before the arrest occurred. So the value of proliferation index in this specific case will reflect all of the following: kinetics of treatment-induced growth arrest, proliferation rate of cells and its time course from CFDA-SE-staining to cell cycle arrest.
Proliferation index is the average number of divisions that all cells have undergone after they had been stained by a cell proliferation dye (e.g. CFDA-SE). Mitotic index is a percentage of total cell population that stain positive for phospho-Ser10/Histone H3 (by other words, the percentage of mitotic cells). Thus, these two indices quantify two very different things. Mitotic index of populations composed of cells arrested in M phase will be high, but the mitotic index of populations of cells arrested in G2 phase will be low (unless arrested in very late G2 phase). On the other hand, proliferation index will indicate how many cell divisions were completed on average per cell (before the cells were arrested in G2 or M-phase of cell cycle, if they were found arrested in G2 or M). The answer therefore depends on specific scenario: if the cells were stained by a cell proliferation dye and then treated with some agent that induced G2 or M arrest, then high mitotic index will support M-arrest as opposed to G2 arrest and the high proliferation index at the same time will imply many cell divisions before the arrest occurred. So the value of proliferation index in this specific case will reflect all of the following: kinetics of treatment-induced growth arrest, proliferation rate of cells and its time course from CFDA-SE-staining to cell cycle arrest.
PI staining alone is good for determination of cell cycle distribution (% of cells in G0/G1, S and G2/M phases), but it cannot distinguish between G2 and M phases of the cell cycle and thus it cannot by used alone for the determination of mitotic index. PI staining alone is also not appropriate for the determination of proliferation index. Proliferation index is determined based on the proliferation dyes (e.g. VPD450, CFDA-SE...) that permeate into viable cells, remain retained in these cells and their content decreases in each successive cell division due to the distribution of the dyes from parental cells into daughter cells. There are other options to determine proliferative status of cells by flow cytometry (e.g. PCNA or Ki67 staining) but they determine other measures of cell proliferation and not the proliferation index defined as is the average number of divisions that all cells have undergone after they had been stained.
As explained very clearly by Roman, mitotic index (based on pH3 staining, for instance) gives you a precise screenshot of G2/M transition. Thus, you could score very specific effects during the different steps of cell division. Indeed the proliferation index is a very general measure of the capacity of cell replication, but it doesn't provide much informations about the subsequent phases of cell cycle.