I own Lowrance only, and do not own SI for the TM, but I do run 3 HDS units. I have had traditional sonar for the TM but just purchased a new transducer that will allow DI on the TM in addition to the traditional sonar. I made this choice after discussion with a number of people who have tried all the possibilities and concluded that DI was the sweet spot for Lowrance hardware. This statement is for "pre Gen 3". I don't know of any new reports on the new stuff yet.
Some say that SI is useful to them on the TM. What you see is definitely affected by the rate of turn of the TM, and one side will be sweeping to the rear while the other side will be sweeping to the front when you make a turn. I decided not to bother with all that, preferring to keep it easy/simple so I could concentrate on my lure and catching a fish!
Makes sense......thanks for all the comments. I am by no means electronics savvy, so I've got a lot to learn with mine. Just ordered everything and it's on the way from Lowrance. I wanted SI on my TM, so the package comes with the spotlight scan, too. I thought I would give it a try.