Antenna tuners. What an interesting subject. Differential T versus conventional T type tuner? Good question. I am not smart enough to give an answer. Theoretically, specs will show one to be more efficient than another. I would be "smart" to say: "It all depends on what antenna is part of the system".
I am a fan of the standard doublet. I have used differential T, conventional T, link coupled, balanced L, and conventional L tuners with a variety of baluns on the input and output side of some of the types.
Given a reasonable antenna, resonant or not, I have worked regular nets (mainly on 160, 75, and 40) interchanging the different types during the duration of the net or round table rag chew. On the receive end, nobody ever noticed that I changed from the theoretical efficient tuner to the theoretical "less" efficient tuner.
I think that commercially made tuners are devices that attempt to match a wide variety of antennas to a wide range of frequencies. In my situation hard to do with a “low” 230 foot doublet from 160 through 10 running 1.5 KW without arcing and sparking
. I am looking for a tuner (coupler) that does it all efficiently. I’m afraid that is not going to happen.
I think it is safe to say that any matching or coupling device will have its advantages and disadvantages depending on the antenna, frequency range, and power we plan on using.
At the end of the day, after discussing apples and oranges, we end up using what works best, given our particular operating environment.
Long story short, buy or build a matching device that suits your needs. I question amateurs who spend $3000 on a transceiver and $50 on a tuner. My operating experience shows that $50 transceiver works better on $3000 tuner than the other way around. I’m not implying one should spend $3000 on coupling systems. For most of us, paying more attention to this part of our antenna system would be money well spent.