First regarding a combi unit; have a look at the Panasonic:
I have a Panasonic HDD/DVD and from the manual it seems recording times for a DVD blank disc will depend on selected recording speed. For example, using SP on a DVD-R, you will get 2 hrs. With LP, you will get 4 hrs. It seems inadvisable to use even slower speeds. Double layer discs give 3min-35 secs and 7min-10 secs respectively.
Not having used this method, I cannot comment on comparative quality of the different speeds. Obviously the slower LP speed will not be as good as SP.