It's difficult to advise without knowing the nature of that hill, steepness in particular.
I don't know much about that Izip model, but the Wisper 905e has been going through a number of upgrades, so make sure it is going to be a latest model before you buy. We have some existing 905e owners who may be able to give more feedback.
Derestriction is an uncommon feature, originally only on the eZee Torq and later a couple of other eZee bikes. The Wisper 905se model has it as well now.
What those bikes have in common is that their motors are internally geared for higher speeds in the first place, then restricted down to legality.
Most other e-bike motors are internally geared for legal speeds in the first place, so derestriction cannot exist on them as they have no restrictor.
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