Just my opinion, but, I would go w/ the 600W hps. unless your just gonna grow under 1 light period. I mean, I would have 2 set ups.. 1 for veg and 1 for flower.. If not, you wont be able to start new plants untill your comepletely done growing your curreent ones .. BY getting the MH/HPS this is what you will have to do.. And that will take way too long between harvests... About 3-4 mnths between harvests is a looong time.. And I can almost guarantee you that once you start, you will want to add a sep veg room after you get some plants starting to flower under your HPS and then that MH light will be pretty much wasted.. Heres what I woud do..
Buy the 600W HPS and just use it till you can afford some cfls... Cfls are hella cheap and you can use your cfls to start vegging new plants while you are flowering plants under your HPS.. And by the time your plants under the HPS are done, the plants under your cfls will be ready to flower.. This will take 1-2 mnths off of your time between harvests.
just my opinion.