Running a HPS and a MH at the same time IS the ideal growing condition, because it gives out almost a perfect spectrum of colors that plants need. Yes it will be more expensive to run both at the same time, but you will make that up in yeild and quality. Plus the more lumens the better, so running both lights means you can have more plants too...
The only draw back to a MH and a HPS, is you need some sort of track system, where you can have the lights rotating in a circle or have two seperate tracks moving each light back and forth and opposite ends of each other in order to give every plant the same spectrum without having 50% HPS grown and 50% MH... You could move the plants like once a day, but that is tendious and may stress the plants a little.
Conclusion, run both lights and invest in a pretty simple track system that will move the lights to give the plants equal amounts of HPS and MH...
!extra note (I just thought of this so it may be incorrect or just not effecient enough) For vegetative cycle you could have the MH on the bottom, and have the HPS above it with some sort of reflectors that directed the light around the MH (make sure it doesn't get too hot or heat up the MH light), or just remove the hood from the MH and have the HPS directly above it say a 6-12inch gap between bulbs. Then come Flowering cycle you could switch the setup. Take the hood off the HPS and put the hood back on the MH and put that directly above the HPS. That should give you a pretty efficient spectrum, with a strong Blue/white in veg. with a less orange/yellow/red, then a strong orange/yellow/red during flower with a less blue/white spectrum, but enough so the plant still recieve lots of it.... This will overall give you at shat load of lumens and a very acceptable spectrum... Please have someone confirm this extra note, but to me it sounds really good..
have a fun grow... my first one is damn fun... almost 5-6 weeks from being finished...
http://www.hempcultivation.com/420/s...threadid=23161