Something with a DD coil the Xterra 305 has a DD available and will get some good depth. The factory coil is a 9" concentric but there are several DD coils available. I use a 5x10 or 6" 18.75khz DD on my 705s most of the time and they have great separation. The 6" DD being the best. I also have an AT Pro and it gets pretty good separation with the factory DD coil. I get good Separation with the 8" coil on my Explorer.
Best to worst for separation of what I have and coil combos
1 Xterra + 6" DD this is absolutely a killer in the trash
2 Xterra + 5x10DD (almost as good) or Explorer + 8" FBS this is more or less a toss up depending on situation and goal
3 AT Pro + factory coil (is DD 8.5x11)
4 Explorer + 10" FBS or Xterra + 10.5 7.5khz DD again a toss up Explorer gets the nod though for depth and ID while the 705 is a bunch faster
5 Xterra + stock coil (is 9" concentric)
The 6" and 5x10 for Xterra have a very narrow field of detection and get good depth with the 5x10 being a little bit deeper.
AT Pro field seems almost as narrow but I do not have a ton of time on it yet, but it is longer so still hits a little bit more trash, but not much.
Both Explorer coils have a little wider detection field as does the 10.5 Xterra coil, the 10.5 Xterra is a tad wider than the Explorer fields but has nearly the depth as the bigger of the Explorer coils.
Keep in mind that there are a BUNCH of other Explorer coils available that have better separation and some say better overall performance, there are also other coils for the AT Pro and Xterra. There are no better coils for separation for the Xterra though. I have heard that the 6" 7.5khz concentric is HOT and gets a bunch of depth (for a 6" coil) but still doesn't the overall separation os the 6" DD and the 6" DD is IMO a better choice because of the field shape, not shaped like a cone.
All of these combos get used when I hunt, again depending on location and desired target with the exception of the 9" concentric on the Xterra which has only been on the machine 3 times since new. I just do not like the coil much.
I do find that the 5x10 Xterra is a fast and forgiving of sweep speed and the AT Pro seems to be also. The Xterra series are frequency adjustable depending on coil choice, 3khz, 7.5khz and 18.75khz. Early 305 and 505, 30 and 50 machines may not take the High Freq coils. You should check if you are interested.
Coincidentally the Explorer will pick out coins that are masked by iron to my other machines, and so far the Garrett seems to disc out the iron well.
This is my experience in my neck of the woods (Eastern Washington).
HH
Jeff