Between the Super Traq, Scorpion and the Simplex..........I would pick the Simplex but with the 9.5X5" coil. Just realize the All Metal mode on the Simplex is just OK. To me it is the weakest part about the Simplex and yes I have owned one. I owned a Super Traq for a long time too.
Iowa Relic was right on point.....a great deal for a very good prospecting detector is the Fisher F19/Teknetics G2+/Bounty Hunter Time Ranger Pro as long as you have the 10"X5" or 5" round coil. The big 11" X7" is useless for gold prospecting. Personally, the Gold Bug/Gold Bug Pro are not as good anymore as the F19/G2+/TRP. Those newer ones have more features, ground balance easier and are sometimes lower priced than the older, overpriced Gold Bug series. Why Fisher keeps trying to make money off of the Gold Bug series name by marketing them the way they do and taking advantage of the unaware consumer is just pure greed. The F19/G2+/TRP have one of the best All Metal modes on the market, they ground balance fairly well in bad mineralization and will hit small gold in the .5 gram to .1 gram range fairly easily. Much smaller than that, you will need a much higher frequency detector. Much deeper than 6" in moderate to high mineralization, you will need Multi frequency VLF=Minelab Equinox, a specialty VLF gold prospecting detector with higher single frequency or a pulse induction detector.
If you are on a tight budget, you could also use the Minelab Vanquish 540 or 440 with an 8" or 10" coil. I have prospected with one and they will do as well as the F19/G2+/TRP, since they are multi frequency and fairly high gain. My 440 slightly outperformed my Teknetics G2+ in the gold fields with the 440 in relic mode (a discrimination mode) and the G2+ in All Metal.