They are definitely different enough.
I owned L'Instant and did not like it, but I love the Extreme.
The EDT has a much brighter and stronger citrus opening (which is very nice) which remains for about half an hour then settles in. Also, the cocoa is very, very faint, but the woods are proportionately equal in their presence.
Maybe L'Instant EDT just dies on my skin or there are some notes I have trouble smelling, but to me it is a very subtle and weak scent that does not offer much other than the really nice lemony citrus at the beginning.
The extreme is deeper and richer and, IMO, the cocoa is done the way that it should be.
I'd recommend sampling it first.