The SE's FE-CO numbers for good targets (general term for non-iron gold, silver and coin items) follow an S-curve from bottom-mid-right screen to top-right screen. The low and high end of this curve is flattened meaning that targets that fall in these areas are compressed (which is why you get a lot of 00-29's for most targets in the high range). Adjacent targets on the S-curve have different FE and different CO numbers since the line is generally diagonal. Targets that "bounce" also, generally, bounce in a diagonal direction (because the scale is not linear). This can be confusing, but having different FE and different CO numbers for adjacent targets may provide more information than otherwise.
The E-Trac is set up so that all good targets are, for the most part, on the 12-FE line. This only gives you the CO number to work off of when deciding to dig. The good thing is that, although the FE number might change depending on depth, etc., the CO number stays pretty much the same. The bounce is vertical instead of diagonal.
So, while the SE may provide more infomation (FE and CO difference in targets adjacent to each other on the S-curve) it is harder to wrap your head around information based on a curve and targets that bounce diagonally. The E-Trac, in effect only giving you the CO numbers with a possible variation in the FE numbers, seems to make it easier to understand. The consistent CO number is a great advantage in certain situations, for instance deep targets. The E-Trac's expanded FE and CO scale (35 - 50 compared to 32 - 32) also provides better resolution.