Attack scope is 1.5x and 6x; Obs scope is 1x and 4x. In theory, the obs scope tilts higher (so you can scan for aircraft before you surface - this seems to be modeled in the game) and also lets in more light so sometimes it's better to use at night/dusk/dawn (not sure if this is modeled in game - I can't really see any difference myself).
The attack scope is smaller and therefore creates less of a visible wake on the surface (obviously important to help avoid being spotted during attack approaches) - not sure if this effect is modeled in game or not. The scopes on the Type VIIs and IXs that I've tried so far all seemed to be set up correctly from this perspective - can't remember if the Type II has both (been a while since I've used one of those).
Good luck and good hunting!
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