Year of Each vehicle? Powerplant of manual Z? What's the throw of the manual? Final Gearing of each? How good are you at shifting?
Based on same model year vehicles, both sporting LS1 powerplant, and both bone stock, I'd have to go with the manual. But that's just me. If you can't shift for s*it, I'd suggest the automatic. An automatic may be able to out-shift me, but not by much. And, I have far more control over my shift points, peak hp/torque, and better gearing. The rest is just experience and modifications.
Martin '01 Formula - MTI Air Box Lid, K&N Filter, Hurst-6, SLP Cold Air Induction & Smooth Intake Bellow Corsa Catback w/Premium Tips '83 V45 Magna