Understanding Frequency Ranges

Sensors

SHFT-02  300 kHz - 1 kHz
MFS-07    50 kHz - 1000 s (best performance not below 10s, better than MFS-06 above 100 Hz)
MFS-06    10 kHz - 4096 s (better than MFS-07 below 1 Hz)
FGSxx    100 s   - DC (Fluxgate starts to perform better than MFS-06 below 1000 s, definetely at 10,000 s)

Cables

5m buffered    1 kHz - 500 kHz
15m buffered   10 s  -  64 kHz
un-buffered    DC    -   8 kHz

Boards

LF  board          DC  -   2 kHz (sampling rate) (better than HF below 10 Hz) (128 256 512 1024 2048)
HF  board          64 Hz  - 524 kHz (high pass filter on) (4096, 8kHz, 16kHz, 32kHz, 64kHz, 128kHz, 256kHz, 512kHz)
HF  board          100s  - 524 kHz (high pass off)
MF  board          1000s  -  64 kHz (128 Hz,  4096 Hz,  16 kHz,  64kHz)

The MF board is especially designed for EMAP and CSAMT where the typical range of your interpretation is between 1 Hz and 10 kHz. During overnight (transmitter of) the MF board can record lower frequencies 10, 100, even 1000s. However: latest at 100s the LF board gives better results.
 

The MFS-07e is a AMT coil with capabilities down to 1000s.
The focus is on 50 kHz down to 100s This is the typical application for mining, minerals and geothermal. The recording time is between 6 hours to 24 hours.

The MFS-06e is a broadband coil. the focus is in the typical MT range 1 kHz (or below the AMT dead band) and 10,000s. The typical application is earthquake, deep geothermal and mostly scientific of vulcanism and upper crust.
The recording time is between 1 day and 10 days.

Choose MFS-07e if you have recordings times around 12 hours, CSAMT or want to measure many sites per day.
Choose MFS-06e if you have long recording times with focus on deeper structures.
The comnbination with MF boards: The MF board is designed for CSAMT and AMT. Typically your record many sites a day. Optionally over night. The MF board has a higher power consumption and is not designed for making low period MT measuremts (the MF has no "sleep mode")

Coils – a Comparison

Often copied is the metronix coil design.

And some coils are cheaper - in any sense. The following two plots illustrate the problems you are facing with some low budget coils:

• strong frequency dependance
• in this case even frequency AND magnetic field input dependance

The metronix coil has a flat amplitude response and is independent from field strength:

The metronix coil has a flat phase response and is independent from field strength: