Interesting post. I also had similar concerns and decided to investigate to see how I could get better performance than my existing Lowrance HST DF SBL transducer (dual frequency 50/200) with another TDX. Main considerations were angle of coverage, available frequency and power (to a lesser extent).
Angle of coverage or cone angle was important to me as I wanted to be able to see my cannonballs and maybe some day see a fish behind my gear

! Ideally a 100 foot diameter cone at 100 feet depth. Manufacturers do measure and report cone angle differently. Airmar is more conservative than some others.
In order to resolve or "see" fish just off the bottom in shallower water, a relatively higher frequency TDX with a narrow cone is called for IMHO, along with a true zoom display function.
Manufacturers of dual frequency transducers for sport fishing (Lowrance Airmar) supply a wide angle cone for the lower frequency and a narrower cone for the higher frequencies. Airman manufactures transducers for the big names in fish finders. I found frequencies of 50,83 and 200 kHz in sport fishing TDX.
Broadband sonar processing produces effective results at lower power levels. Good thread on this on The Hull Truth web site.
Bottom line, I went with an Airmar P66 transducer, 50/200 kHz, 45/11 degree cone angle, 600 watts power. Processing and display is done with a Lowrance HDS8 GenII. Have used it twice, performance is remarkable, big improvement over previous transducer imho. Investment was minimal $130 from Airmar plus freight. Able to see cannonballs, good resolution on bait schools etc.
Hope this is useful to you.