I've used both, but don't claim to be an expert on either.
I prefer the E-trac for finding targets (both target ID at depth, and the ability to separate targets and locate targets in the same hole with trash. I also prefer the noise cancellation (there is one area I can't even operate the V3 due to EMI issues, where the E-trac was fine) and the fact that I don't have to care about ground balance with the E-trac.
I prefer using the V3 target analysis and pinpointing once the once a target has been located. I find the pinpointing easier to use (tho the E-trac's is certainly usable), and I like the V3 telling me which frequency is dominant. I also prefer the V3 spectragraph system to the E-trac system. Finally, the V3 is easier on my arm (it never seems to get tired), where my arm tires easily when using the E-trac (I imagine this could vary person to person). I also really like the V3's target ID with no tone on rejected targets -- this is a sweet feature I miss on the E-trac.
In limited testing with both machines in the field at the same time, I found no target the E-trac saw that the V3 didn't (and vice versa), yet the E-trac target ID was much cleaner. I didn't find any targets deeper than 10 inches with either on that day, however, it was a very limited test. On the targets found in holes with other trash with the E-trac, I'm not sure the V3 would have ID'd them, tho I didn't have the machine along at the time to try that test. I have found one large copper with the E-trac at a site hit hard with careful gridding with the V3, at 10 inches with iron in the plug, but there is no guarantee that I actually passed over that target with the V3 (its probable). Overall, I have confidence that the E-trac is hitting the deep targets, and worry about the V3 past about 9 inches. As Detector alluded to, this could be a skill issue with the V3. I do run both machines pretty much as hot as possible, and the E-trac seems more stable.
I've had bad D2 coil issues with the V3, OTOH, White's customer service has been excellent.
As an overall summary, I prefer the E-trac on a technical basis, and the V3 on an ease of use basis. On the day I had both machines out for a test, I morphed into locating targets with the E-trac, and recovering them with the V3. I have yet to decide which machine to keep. I wish they would build a machine that combines the best of both
HTH