Yes, get a 600W HPS for flower.
However, T5's are the BEST when it comes to VEG. They create a very tight internode length.
If possible, get both. IT is well worth the investment. You just simple don't get the tight nodes with any other light as efficient as a T5. However you wont get the bud DENSITY or SIZE without that HPS, idc what anyone says, if they tell you different they are full of shit. This has been tried and tried and tried again and it works.
No, I'm not saying "listen to me, listen to me" I just despise misinformation from people who obviously have not clue what they are talking about.