Difference between band pass vs. low pass + high pass?
Is there any difference to simply using a band pass filter of 0.3 – 40
  Hz?


No - but some filter design tools might not provide an option to design a band-pass filter with a unity gain "plateau" over a range of frequencies (instead, the response of the band-pass filter will be a bump at a given frequency).


  why the notch at 50 Hz when everything 40+ Hz is thrown away, anyway?


50 Hz is dangerously close to 40 Hz, and might still be in the transition band of the filter. The 40 Hz filter would have to be very steep to entirely attenuate the 50 Hz. It would be much easier to remove the 50 Hz component separately with a steep notch filter.