Like rgchan, I'd rather play decent opponents in a nice money added than poor players in a freeroll. However, my primary reason isn't concern about the donks play making them hard to read or about being sucked out on by people playing rags. It's EV. Once I factor in the amounts of freeroll prize pools and the sizes of the fields, I make a fair bit more money per hour playing the buyins.
As a hypothetical comparison, take a $2.20 buyin with $20 added. Many only draw about 15 entrants, so if I'm an average player, my EV is about $3.30 for a net of $1.10 per tournament. In a $50 freeroll, if I assume "only" 150 entrants, my EV and net EV are both 33 cents per tournament. If I assume I'm decent enough so my adjusted EV is three times average, that puts me up to $1.
That seems almost the same, but it doesn't factor in the average length of time I play - which is obviously a lot shorter in fields of 20 than 150.