The HUGE difference is that CM7 is Gingerbread, while CM9 is Ice Cream Sandwich (more or less, as both are running on a Froyo kernel).
So CM9 brings most of the ICS's goodies: better battery life, better interface, optimisations, etc. and a few still unsolved bugs.
I'll try it too this weekend or the next (depending on the available time).
Nokia 5110 > Alcatel OT303 > Nokia 3310 > Nokia 1100 > Nokia 1112 > Nokia 5000 > Nokia 6555 > Nokia 3120C > Nokia E51 > Nokia E52 > Motorola Defy > Galaxy Nexus > Nexus 5
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