difference between 210/5.6 Super Symmar XL and Schneider Super-Symmar HM 210mm f/5.6

except for it being cheaper?

Primarily the 210XL has massive coverage (over 500mm) vs 210HM having 356mm coverage (ie 8x10 inch). The 210XL is much bigger and heavier for this reason. One of the features of the Super-Symmar HM series is that they give a better performance without having to stop down so far (IIRC f/16 is optimum) but of course this is not always possible on say, 8x10 in the field.

The Super-Symmar HM is discontinued
The Super-Symmar XL need a center-filter (nearly as expensive as a used decent 210 HM) to avoid objectionnable vignetting when generously shifted
The Super-Symmar XL is supposed to have been cured of Schneideritis.

what is scheideritis?

For Schneideritis, search the forum.

This thread http://www.largeformatphotography.in...&highlight=210
is about supposed Schneideritis of a Super-Symmar 210 XL - or it coulb be outgassing of paint.

I have the 210mm SSXL and it is one HUGE lens, with a metal(!) front cap. Like the other SSXL lenses the front element is much bigger and heavier than the rear (unlike say, the symmetrical Nikon 150mm SW), giving me occasional nightmares about the lens falling off SPLAT off my camera!

for anyone who does get this fine optic, always make sure you tighten the lens board holder! ;)