The comparison between G.E.B. disc and TR is obvious: the former is motion and the latter non-motion. The non-motion TR mode allows you to get right in next to trees and other obstacles as well as right in next to other trash items. GEB disc would just blend the two objects together and give you a broken signal at best but usually just completely masks the good target, especially if the trash object is larger than the desireable target.
The comparison between G.E.B. "Norm" and TR is somewhat more complex. Mainly the difference is in target separation particularly in areas that have a lot of iron trash. GEB Norm is basicly unusable in areas that have lots and lots of iron trash, like three or more signals per sweep. Yes, you could detune and try to ID each target using the meter, which I do in areas with moderate trash, but in high trash areas, it's just too time consuming and labor intensive. Using the TR mode it's possible to use a little discimination and eliminate the iron trash objects without losing too much depth (depending on soil mineralization, of course). If you're willing to lose a lot of depth, it's possible to run a high discrimination setting.
It is at those locations with an extreme number of iron trash items that TR is most useful. Yes, you might only get 3" depth but that beats 0" which you'd get not being able to use GEB norm at all due to the high trash level.
I started detecting in the late 70's with a TR machine so your experience certainly goes back further than mine. But my main point is that after not using TR mode for at least 8 years (my time spent wandering in the "XLT desert"), I'd forgotten that in some situations, TR is VERY useful.