The 600 watt HPS will be a better long-term choice then three T-12's. I highly recommend a hardening off time moving to the HPS, a period of using both lights. I would put both lights on timers, the T-12's at 12 hours a day and the HPS at 30 min plus rather high above the cuttings (36 inch or so). Check the cuttings daily for moisture levels, and increase the photoperiod of the HPS by 15 to 30 min a day. Watch and listen to the reaction of the cuttings, if they start to show signs of stress, either raise the light or cut the photoperiod back a little. Then stay stagnet for awhile untill the cuttings regain their health. Once you get to 10 hours a day, you can turn the T-12's off and continue to increase the HPS to 12 a day. You can then slowly lower the light if you want, pRobably no need, the bulb should be strong enough and you will need room for all of the new growth.
Once spring comes, you will still needed to harden off the cuttings to be outside.
Also, HPS and MH bulbs loose around 25% of their intensity by 6 months of use at 12 hours a day and then slowly dim until they burn out. I would start with a new bulb, howevef if you don't then replace the bulb later you will need to raise the light to compensate for the extra intensity.
Good growing, Scott