What do you mean by "totally manual", that you have to calculate f-stop from distance and a Guide Number each time? The SF20 has a little electric eye on the front that reads the reflected light and quenches the flash based on the ISO and f-stop you input to the flash, like many so-called "non-TTL AUTO" flash guns.
I found a used SF24D for about $150 and like it better than the SF20 (I have M6, like the MP has no TTL flash capability). The reason is the SF20 lets you input only 2 apertures (2.8 and 5.6 IIRC) whereas the SF24D lets you input all full stops from 2.8 through 11).
