Actually, ive grown several crops from MH alone and it still grows killer bud. HPS is preferred because of the light spectum is "warmer." (reds, oranges, reflects the autumn season) I have found that using HPS alone has given me bigger yields but no quality differences. Natural sun light vs. HPS light, I don't believe should reflect any huge differences. The only thing to be concerned about is that the plants when moved outside will receive proper lighting. I.e. are they in the shade or full sun, for how long in each during the course of the day. OVerall its' worth getting them outside to get fresh air since you don't have a proper ventilation/cooling system setup. If you can't vent the air out, have you thought about a portable ac unit? Do a search on the net for one. Kind of pricey ($300-500) but it can lower by 20-30 degrees. Just plug it in and vent the air into another room or through the window. If I was you I would do both, (ac unit) and a vent fan with a can filter for odor control. Heat will seriously fuck up your plant believe me! Anyway I wish I could grow outside some day not having to worry about all that shit. But then again i don't feel like having a cell mate named Bubba .
Happy growing.