You could easily conditionally suppress the sub reports based on the user selecting a parameter. Just set up your paramter the way you want for indicating grouping or not (like 1 for grouped 2 for ungrouped).
Create your 2 versions of the reports as sub reports and place each on a seperate header. Go into Section epert and set each to be conditionally suppressed.
suppress formula section1 as : {?parameter}=1
section 2 as: {?parameter}=2
or any other variation of this same process would work.