I am also one of the users that keeps the Gfit “auto-tracking” enabled. I find it pretty good and with no real negative impact. The battery is pretty good, steps are not exactly the same but so close to be irrelevant, and I’ve never experienced ( or noticed) a wrong tracking of an activity. The 15 mins delay is a good point, but I guess I don’t really care too much since the activity is “light”.
At least for me, I keep the SA for all the “real” activity, and the fit more as an overall aggregator for all the tracked and non-activities and the smart scale measurements.
After the significant improvement with the last update, I use the SA much much more, but I believe I will continue using both in the soon future.