Here is a youTube video comparing the 340, 440, and 540.
The 340 appears to have some decent features. I recommend making a test bed where you live by burying some coins, brass, pop can, pull tab, etc. far enough apart from each other, then run the 340 over them and note how it reacts. Adjust some settings and try again. Repeat until you get a handle on it. Then maybe go to a local playground/ball field where there is bound to be some buried coins and junk then see how you do. Practice, patience, frustration, and hopefully some neat finds are all part of the hobby.