The SL is a great little detector with almost as much depth capability as the TDI. I prefer to use the SL simply because of the weight. The big reason for the depth difference is the battery voltage. Many people have tested the regular TDI against the SL and found little to no difference in depth capability.
I use the RNB battery system but just purchased a Reid 4 li ion cell setup for the SL and will be trying it soon. This should put the SL on par with the TDI depth wise as far as battery voltage goes. The Reid system can be found on ebay now.
Now, without going into details, I can say the SL is basically designed the same as the TDI including the gain, so the two models are very close in overall design except the SL is designed to use different types of batteries. The big reason for the mod was the difficulty in shipping LI Ion batteries. This is dumb because there are fewer Li Ion batteries in the TDI than in a laptop computer and they are allowed to fly.
Regardless, the SL is a great little detector.
Now, unfortunately, what you determined when trying to eliminate nails also eliminates 3 ringer bullets is true. Since such bullets are rare out here I haven't spent any time experimenting with the delay or other controls so see if I can separate the two. I just usually listen for the double blip you get with nails, but not with non ferrous items. This double blip idea works for many nails but not all and also requires you circle the buried target and listen to the signal from different directions.
Now, I have experimented with some different ideas including one that allows you to operate at perfect GB but also sense what targets are if the GB was altered. This gives both depth and the ability to tell many targets without having to adjust the GB each time. I have already experimented with a different coil design which also adds depth.
Whether any of my ideas ultimately are available is yet to be determined.
As for suggestions, well I will say, practice and experiment with the detector. It takes time to really appreciate what can be done with the TDI or SL and many people lack that patience needed. It takes months to get a real idea of what can be done.
Reg