I bet a 50% fuel engine will almost always win this as the production capacity of the harvesting station will vastly outperform the fuel cost of delivering the station to it's destination.
I guess it depends on the relative size of the tugs and the station.
A 50% engine uses about 75% more fuel per EPH than a 40% engine.
A tug using 50s instead of 40s will reach the destination in 20% less time.
In that amount of saved time, will a station produce that much extra fuel?
My slow tug uses ~557kl/yr.
My fast tug uses ~979kl/yr.
Both are roughly the same weight as my harvester station, which has ten modules and can produce about 600kl/yr (with my naval admin mining bonuses accounted for).
Suppose it takes the fast tug a year to deliver a station, and six months for the return trip.
Then the slow tug would need 15 months to deliver and 7.5 months to return.
That's a 1.5-year round trip for the fast tug, consuming ~1.47ML of fuel.
The slow tug takes 1.875 years, consuming ~1.04ML of fuel.
So the fast tug needs 0.43ML more fuel. What does the faster trip gain me?
Just 3 months of fuel production from the station.
In that time, the station produces ~0.15ML of fuel.
The slow tug is better.
After all is said and done, delivering the station with the slow tug leaves me with an extra ~0.28ML of fuel.
But if my tugs were much smaller, the faster tug might be better, because the 20% time savings equals much more time, while the 75% extra fuel usage equals much less fuel.
If you cut the tug sizes in half, then the trip takes 50% longer and uses 2/3 as much fuel.
The station would then be harvesting ~0.225ML more fuel when delivered by the fast tug, and the fast tug would be using ~0.29ML more fuel per trip than the slow tug.
Cut the tug sizes in half again, and the faster tug will be the better option.
Of course, all this does not factor in the value of finishing the return trip sooner.
If you are going to build stations as fast as you can tug them all away, then using faster tugs means you get more stations built and delivered sooner.
But it costs you more fuel to do it that way.
If you are going to build stations at some given rate, then you will want to build enough tugs to handle that build rate, so that you always have a tug ready when a station is completed.
This will require 25% more slow tugs than fast tugs.
But the fast tugs are about 30% more expensive, so this really only saves ~2.5% of the cost of tugs for this job.