You can think of the Multi Kruzer is the waterproof Impact, minus many of the modes. The Kruzer is also a little lighter, and has the newer 11" round DD as standard.
Also, all modes of the Kruzer and Anfibio come standard with the wireless headphones.
The Anfibio Multi is like a blend of the Impact and Kruzer. Collapsible shaft, stick handle on straight stem, 99-tone mode is back, and a couple of new exclusive features such as 5-tone mode, and the quick lock sliding armrest. The Anfibio also has the back-lit keypad, which the Impact and Kruzer series don't have.
Kruzer and Anfibio share coils. The Impact coils are the same from a technical specs point of view, but the cable is shorter.
To help a customer select the right model, I'd ask a few questions:
1. Do you need the whole detector to be waterproof?
2. Do you prefer S-bend or straight stems?
3. What is more important to you, overall weight or balance?
Sometimes 2 & 3 are more easily answered by feeling the units in person and giving them a quick swing.
Hope that helps.