DVD-R Although older is just as common as DVD+R, I doubt your families DVD players will be unable to run DVD+R movies, unless they bought their player just after they came out
From what I can tell, there is absolutly no difference between teh two types except how they originated (Two different brands) so quality shouldn't be an issue.
Also, I highly doubt using firewire over USB 2.0 will have any difference at all to the quality unless the USB cable cannot stream fast enough (which is not likely either)