tl;dr – just make them combi-weapons, then add the "gamble" factor back in by allowing obliterators to spread out attacks that would be sub-optimal against a single unit, and letting mutilators get an extra attack for using different profiles rather than sticking with the best.
In terms of balance, Mutilators are basically unchanged, except they trade peaks and valleys for a more stable selection of options, with a +1A if you go back to peaks and valleys.
Obliterators actually do less damage on average than their original incarnation, but gain T5 from the new version without going up in points. Compared to the new Havocs, they match up reasonably well; for slightly less than the price of two Havocs with missile launchers, you get a single model with an extra total wound, no loss of firepower as it suffers wounds, beefier melee attacks, and a 2+/5+ save. The downside being that the Obliterator has about three-quarters the total firepower, those extra wounds won't help against multiwound weapons, and it's very slow.