It is true that PMMA is soluble in DMF & in chlorobenzene but the 2 solvents differ from each other in polarity , in boiling points and in surface tension. DMF is a highly polar aprotic solvent with a boiling point of 153 oC while chlorobenzene is classified as slightly polar with a boiling point of 132 oC. You know that slightly polar molecules usually have a much lower surface tension than highly polar ones.
These 3 properties will be important factors in getting uniform films by spin coating or non-uniform films together with other factors.
It will be worthy to use a mixture (chlorobenzene/DMF) at different ratios to see what quality of film you will get at the same speed that you applied for each of the solvents separately.