The X35 coil can use frequencies from 4 to 25khz, the high frequency coil from 14 to 55khz. The X35 is deeper on silver but at 25khz is still pretty good for small low conductors like nuggets (the Gold Bug Pro operates at 19khz). If you put a quarter and a pull tab on the ground at the same level, at high frequency to tab is much louder. As you lower the frequency, the tab gets quieter and the quarter gets louder. You can also get a boost in 4khz. The X35 can also normalize ViDs so all frequencies have the same numbers for the same targets if you want, which the HF does not do.
The HF coil is more limited with 14khz to start with although many really good detectors operate near this frequency (T2, Racers and more). For gold prospecting, it is probably more useful and some say it separates better although I can't tell.
I think the X35 is more versatile but that is my opinion. I have both but use the X35 more. I don't think you can go wrong with either.
