The HF DD coil that I have is the 10.5" round, which I would imagine will go a small amount deeper than the 9"x5" simply due to it size.
As far as which one goes deeper between the HF and LF? It depends on several factors, not the least of which would be mineralization. In average to mild conditions, the LF coil will see coins and relics deeper, but will be more likely to miss gold or fine jewelery (chain, earings, etc...).
The division of bin segments is wider at the top of the scale with an LF coil and narrower at the bottom. While the HF coil has wider bin segments at the lower values, and narrower at the high end. All of which (with all else being equal) lends the LF coil more accurate at identifying coin value range targets and deep relics, and the HF is better with gold and most other small targets, generally speaking. The MF coil has the most uniform bin segments throughout the range, so is a good one to have as an all purpose coil, or as a bridge between extreme conditions.
Since I'm personally more of a coin and relic hunter, I prefer the LF coil whenever I have conditions that allow it, but will use the MF coil if possible when the LF isn't appropriate. The HF is more for extreme minerization conditions for me, and lately I've been running into more places that I'll use it......but only because have to. If I was a prospector, or primarily a jewelery hunter, then the HF would be the logical choice.
NOW IF ONLY MINELAB WOULD CUT LOOSE A 3kHz DD COIL......!!!!!