In all fairness, I have Owned Both So what I can say is the TDI Discriminates Better, Because with the GP3500 you get a Low-High-Low sound on Items of value but after about 6" that can change to High-Low-High Sound and Vice Versa, But Once it gets past 6" it Gives a Good Signal on Bad Items telling you that they are Good,
With All TDI Models It will Report if an Item is Low Conductive or High Conductive depending on where you have it Set, Or it will Report Both when switched to All and The TDI's will Do This At Maximum Depth, Not Even The Top Brand's of VLF's Can Do That, It Knows what it is As Far (Deep) As It Can See. Also you can set it up Hotter by off setting the Ground Balance and you can Also use the Ground Balance to Discriminate OUT Unwanted Items, It is Not A Perfect System But it Works,
Along with the Manual Ground Balance you Can Switch it Off Completely which Puts it in All Metal Mode Boosting the Depth capability Even More where Coil Control is the Key to using this method In Mild to Medium Ground,
If you have Switched to Low CON, then all High CON Items Either Do Not Make A Sound or they will make A Noise like a Hot Rock does on A VLF and Only Signal After The Coils Has Past The Object and By Switching From Low To High and Vice Versa you can Tell what the Item Is,
Although The GP3500 Has A Nice Auto Track, The TDI has A Manual Ground Balance, If you are Hitting Hot Rocks you can tune them out either by Adjusting the Ground Balance OR, If you Raise the Delay From 10 to about 13 you Can Make them disappear, which also Helps Knock Out small Iron Items such as Nails Etc,
So in junk Filled sites the TDI's Can Be A Real Bonus, also when you Hit Nails if you are in Low you will get a single Wooo if the Nail is pointing North South of the Coil and If it is Pointing East West it will Go Wooo Wooo and Vice Versa If you are In High and it does this with All long Shaped Objects.
Because I always seem to end up In Junk Filled Site, (Bugger It) This is Why I changed to the TDI SL, I loved my GP3500 But They Are Bloody Murder In Such Places and If there is EMI or Storms within about 500ks it sings like A Canary, When I was testing My TDI SL out in my Home In The City I can Turn it up to 9 and it is Smooth, If I take it out Side I can turn it up to 10- Flat Out, And Being able to turn it up to the Max and it remaining Smooth Allows me to Concentrate on Listening for Faint Signals. With the Standard 12" Duel Field Coil it will See Targets Down to 0.08 to 0.10 grams or 0.06 ozt and with Smaller Coils it will See Smaller Targets To.
My GP cost me $5240.00 AUD, My TDI SL cost me $1552.00 AUD.
Anyways I hope This Helps,, John
