Two Lighting Questions

Howdy all. I'm on my first grow and my seedlings are taking off quite well thanks to the info on this site. I have a couple of questions though.

1.) I'm using a 150w CFL growlight right now, growing six 50/50 sativa/indica Crystal Rain seeds. I've read and seen that the light should be left on for 24 hours a day, but I've read posts here where people do 18/6. Is it better to do the 18/6 or 24?

2.) I'm thinking of upgrading to a 150 HPS light for flowering. How much heat does a 150w HPS generate? Just asking because I want to make sure I upgrade my ventilation beforehand if I get one. I currently have a 150cfm inline duct fan for intake, and a 150cfm for exhaust going to an activated charcoal filter. My grow hut is in the attic, so bringing in air from the HVAC ducting is a must because of temperature fluctuations.

Thanks for the help. You guys are great.

24/7 is what i use. it lets you get a head start cuz the plants just take off immediatly. with 18/6 it just takes a lil longer to veg. you ventilation system should be fine. 150w hps. is not going to produce a TON of heat. but def more than your cfls

Ive always use 24/7. Works for me. Some people think the 6 hours off is good for the plants. Its a matter of what you like.

the 24hr thing seems to be working good for me. I was just making sure I wasn't making a newbie mistake.