Hi Jim,
It all depends on how much time you want to save and on what probabilities you're working on. As a
general rule of thumb,
the Infinium in reverse disc will:
1 give a low-high signal on medium to large pieces of iron,
2 give a high-low signal on small bits of iron,
3 give a high-low on large gold nuggets (unless deep, than it is unlikely to give any signal at all because you loose a lot of depth in reverse disc)
4 give no signal or a very faint signal (a blip type of signal) on small gold.
In trashy areas, because there's so much junk around, you probably only want to use the Infinium to search for targets that give a faint high-low or no signal at all in reverse disc. There will be too many signals that will give a strong high-low signal (large gold or smallish iron) and low-high signals (large iron) in reverse disc! But that's where most of the large gold is left behind, so it depends on how much time you have. I've never used any other detector in trashy areas other than a GP 3000 and the Explorer SE (I didn't give that too much of a go in these areas) so I'm only generalising, the 3000 wasn't too good in trashy areas but I was still able to find small gold, you just have to dig the good signals. I have never hunted for silver.
The other thing is, you can't walk around with the Infinium in reverse disc. You put it in rev disc when you want to check the target then decide, then put it back to your normal setting and move on, whereas with VLF discriminating detectors, you can. But VLF's struggle in goldfield type grounds, they loose a lot of sensitivity - so I suppose VLF's will still be alright for larger pieces. They do very well with silver, I know that.
I have heard of people using (one guy I think) an Excalibur in trashy areas and doing well, I can understand why, but I think a BHID would be better, though may be noisier - it just goes on and on
PI's do however give you info just from the target response, and the Infinium will do that and then will give info in rev disc - that is - it will allow you to analise the signal
yourself, and in reve disc it will allow you to analise the some more - where the VLF's will analise the signal for you.
The knob that you use to put the Infinium in rev disc is the same knob that you use for "discrimination". It is numbered 1-9, and where 10 would be is reverse disc. If you're looking for silver, it would be wise to use a sample that you found in the same area, (wave it over the detector) and try to find the point on the discrim knob where it will give you a signal that's both high-low and low-high. I call this a warble and it is a very distinct sound. You won't be able to find this setting for most items (so it wont be affective to look for them this way), but I tested a dime and a US 1 cent piece and I found that in the 7 and three quartes setting on the discrim knob the signal warbles. The reason I mention this is because the silver nugget in the third pic that you posted in this thread will give you a warble (a very recognisable signal) on a setting of 7 and three quarters on the discrim knob - BUT, you will be loosing a lot of depth. The higher you turn the discrim knob the more depth you loose, so those nuggets would have to be shallow for you to identify it this way.
Silver (at least the ones I have at home, like silver rings and other assorted trinkets) in rev disc will give a fairly good signal still (mostly high-low but a large ring gave a good low-high), but you will loose some depth although not nearly as much as with other items that aren't iron. (when you put it in rev disc).
I haven't used a Whites TDI but I think it has a similar feature as the Infinium's disc knob and it might even have a wider range (which would be good).
I'm not sure if I've answered all of your query, but if I were you, I'd also look into using a Sovereign gt (same as Excalibur) or a Whites BHID. The Sovereign will probably be quieter but you'll be able to search faster with the BHID - in theory!