OK, so IF I get the nova blast off, 12 shots, 6 hit 1 rend, which is also the only wound.
ION gun, we have 3 shots, 1.5 hit, of which we cause 1/2 wounds, no save.
BUT, we only get rending if the Nova goes off, which only happens 2/3 of the time, 1/3 of the time, we actually cause us ourselves a wound.....hrm.
PLUS, I'm not using that Nova for something else, like 3++, or ripple firing the secondaries.
I think the Ion is still better.