2cp rupture = 15 energy spent, 60 energy returned if you keep poisons on the target (will need to switch back to reapply once), for a MAXIMUM net gain of 45 energy. Alternatively you could say it's 25 energy spent with 70 energy returned. same thing, really.
5cp rupture = 0 energy spent, for a net gain of 105 energy, but you have to re-apply deadly poison 2 more times. or 25 energy spent with 130 energy returned.
Assuming your values of 12k per melee hit, and 2 hits to proc, you're looking at +45 energy, -96k damage for 1, and +105 energy, -144k damage for option 2. @28.8k mh mut, and 14.4k oh mut damage, your mutilate DPR is 0.785k damage-per-energy. The energy gain in option 1 is therefore worth 35.3k damage and the energy gain in option 2 is worth 82.4k damage.
In both cases the lost 2 autoattack+1DP damage per poison reapplication is more than damage from the extra energy for mutilate. Ideally you'd want to take a weighted average of DPR of all of your energy-costing abilities to get a closer-to-the-mark estimate. Going based on mutilate DPR is admittedly worst-case because the only rogue combo builder with less DPR than mutilate is revealing strike (sinister strike probably also does with haste-focused gemming).
Last edited by shadowboy; 2013-05-18 at 04:29 AM.