All these backs use the same sensor but with difference electronics.
Aptus 75 is very slow, connects with FW 400, and has a dim LCD.
Aptus 75s is faster, also connects with FW 400, more noise than 75, same LCD.
Aptus II 7 is as fast as 75s, lower noise than 75, better LCD, connects with FW 800, better color rendition.
H4X connects with Aptus 75, but not with Aptus 75s or Aptus II 7.
If you can get the II and it is not much more expensive, it is the best way to go.
Next priority is 75.
75s is last priority IMO.