Get the hps, you wont be sorry as long as you cool it properly. Personally I'd put 2x 1000 watt hps in that 8x4 an call it a day. I dont like 600 watt hps... that's where I started because people on this forum preach about them all day long.. but you'll get bigger buds an better results with the 1kw bulbs very very easily.
And honestly who cares about another 60$ on the power bill when your yielding 50% more.
Dude I get you man, I do. But I can't keep those fuckers cool for the life of me. Two 600 watters? Easy to cool with central air. Two 1k watters? You need an extra A/C in the same room and then you have to dump the hot air from the A/C somewhere too.
Huge buds that can knock someones eye out? Ancient Astronaut theorists say YES!
Hot as hell with fire coming out of your tent? Satanists say HELL YES!
All joking aside they can be cooled. But @ an extra $100 per month for cooling alone. Those babies are HOT