The K version was changed again, being 8 elements in 7 groups, growing a bit in size from the M42 ones.
The SMC-M one changed again, being 5 elements in 5 groups, and shrunk back down to accomadate the standardized 49mm filter thread. Definitely not as sharp as the K series.
I think out of all the lenses I've owned, there was the most variation of the 28mm f/3.5 from it's beginnings as the massive 58mm front Super Takumar, through the smaller Super Takumar, S-M-C Takumar, SMC, and SMC-M. It was changed fairly significantly with each generation.
I think the K one is the sharpest of them all, and I believe it's generally regarded as the best of that lens by most.