My experience has been that there are really a few splits.
1) Tankers: If you just need fuel and the ability to move it around, dedicated tankers are the answer. The good news is that these can be small, fast, and fairly inexpensive. I use them all the time for use in fleet trains and in moving fuel from colony to colony or bases. Use them all the time.
2) Harvesters: These are going to be big slow and expensive. No way of getting around it. To be honest, I tend to use these as a "set and forget" type of ship if they are just harvesters. If they are pure fuel barges, they can be slow and it really doesnt matter. Let them scoop and dump the fuel in a system. In many cases, where I have these running, they are dead last priority to upgrade. They just work. I honestly use these fairly often when I have a colony system with sorium gas giants. Its cheaper than building refineries, and lets the workers focus on stuff like construction factories first. A couple of harvesters, and a colony with a fuel transfer station and your set for fuel in the short term. Once system infrastructure gets built out, and the population can support its own refineries, I move these along to the next frontier colony. If the colony has huge amounts of sorium in gas giants, then these things tend to stay and they just run until I remember to upgrade them.
3) Mobile support: So these guys are purely situational. These are big, slow, very expensive but have massive fuel tankage, harvesting, fuel transfer, and large supply bays. I use them as "one stop shops" for ships out on the front/fringe of explore space. Instead of my scouts/surveyors having to run all the way back to a colony, they stop off at one of these ships for fuel and stores and then back to work. Honestly, these ships are purely situational. I use them when I have big long warp chains that have systems I dont really want to drop colonies in. In those cases, its worth it to drop one of these in a system with a gas giant and just leave them. Their duration times are huge, so they dont move often.
As far as efficient, it depends. If its just moving fuel around, tankers. If its skimming sorium and dumping it on an established colony, just basic harvesters work, and they arent too expensive, especially if fuel transfer is available from the colony. That kind of ship is harvesters, fuel tankage and small engines. They are slow, but comparatively cheap. My "set and forget" harvesters fall into this catagory.
As soon as you get into harvesting, refueling, and tankage the cost of the ships goes way up, as does the size. They are efficient, and in terms of overall expense, they can pay for themselves over the long haul as they dont need to be updated very often, but they are big and expensive and slow to build initially. They will need to be updated when engine tech changes, just to keep up with the frontier pushing out, but arent high priority.
Mobile bases or the "one stop shop" approach are NOT efficient. They are big, expensive, and need to be updated and resupplied regularly. I dont use these unless I really have to, and thats dictated purely by terrain. i.e. lots of poor/no mineral systems in a long chain. Otherwise, I try not to build these.
As far as tugs are concerned, I dont use them for harvesters very often, I think the dedicated ships are more efficient. Ditto for tankers. For the big mobile bases, then yes, the tugs are actually beneficial. Especially if they are going to be long term deployed bases. Getting rid of the engines and letting the tugs do the work is much more efficient. Engine failures eat up a ton of supplies, so getting rid of them cuts way down on the maintenance supplies consumed by the base, which extends their deployment time. Especially if the bases have shore leave facilities, at that point they become so big that tugs are definitely the way to go.
I tend to build fewer, but larger tugs rather than lots of small ones. I think they are more efficient, and it cuts down on the micromanagement. Tugs are for big projects, otherwise I build ships.
