Historically, Perkins loans have had the more favorable interest rate (a fixed 5 percent) and that is why they were need based.
It has only been in the last few years that the variable rate Stafford has been lower (the lowest interest rates in general in the history of the U.S.). By next year, it is very likely that Stafford loans will have a higher rate than 5 percent and it is doubtful we will see Stafford loans having rates lower than Perkins loans in the near future.