As for which is better Range or Speed: I think it depends very much on which kind of targets you are going after. Travel time is the biggest time element of Exploration so range is probably superior since it can cut your travel time by 50% (or more!) in some cases. Scanning usually takes up the last 5-30% of travelling around a system, so even doubling your scan speed will only speed up your earnings by a marginal amount at the end of hop between object. Scan speed is only really superior in systems where the rich targets are lumped up close together near the star and already in range of the normal DSS, like M class red dwarf systems with a clutch of terrestrials/terraformables that are all within 7Ls of the star.
Of course, if you're the kind of person that likes to get close up to each object you scan, then the Fast Scan is probably the way to go.
That was my thought process, to a point.
I only DSS full systems, don't cherry pick (just for RP reasons), and as you say, travel time is a big element, but bodies do tend to get bunched together, either as twin planets orbiting each other or gas giants with a bunch of satellites. Most of the time these days though I don't bother with systems with a ton of rocky / icy bodies, so as I said, next trip I will try out the range mod.
I did like the fact that the actual scanning didn't feel interminable, but I'll be interested to see how much further out I can be before it starts to scan. The fast mod sacrifices a bit of range, so in some cases I needed to be within 6 to 9 Ly of the target for the scan to begin, I'm assuming that the long range mod should start to scan around 20 Ly out.