At risk of telling you more than you wanted to know....

Apart from dry film issues, which I suspect are neglibly different, an
important difference in these two plating baths is that the Methane Sulfonate
baths will not dissolve the Stannic Tin oxide that inevitably forms when Tin
is plated, and it forms a milky white dispersion in the plating tank.

When the metal is plated, the oxide tends to co-deposit.  This oxide then can
create problems in fusing, if the boards are fused later on.  Clearly, this
is not an issue if the Tin is simply used as an etch resist.

Rudy Sedlak
RD Chemical Company