For the best performance on bad ground you need the GS 5 which has the edge over the Whites T.D.I.
If its low or even medium mineralisation you would do better with a single frequency machine. I used a Tejon with the 12x10 inch widescan coil which could find everything the pulse machines did but easier.
The E-Trac doesn't seem to have any extra depth over the Explorers except for the XS (assuming all fitted with the same coil). If its deep in your area it would be worth considering. If your in an area where multi-frequency doesn't give an edge then I would use a good single frequency for the faster recovery speed.
Normal P.I. performance rules don't apply when you use ground balance to provide a degree of discrimination.