First, what is the average length of the reports you type? If you do a large number of relatively short reports and/or have a lot of normal dictations, per report might be better. If there are a lot of longer reports like say MRIs, PET scans, etc., maybe per line might be better.
I personally wouldn't consider an hourly rate because I can produce more than 100 radiology reports in a day. I make $1.50 per report. That's in the midwest and my account is a smaller radiology department in a smaller hospital.
I would suggest you look at your workflow on an average day and do the math. I think you'll find per report is the best rate for radiology.