have you grown in an attic before? temperatures fluctuate wildly. on a hot day attics can get to be like 140 degrees without proper ventilation. if you can control temp in your attic youll be fine, but im going to guess that you wont be able to since you said it gets really hot and summer is forthcoming.

the two 600's sound like they will work very well in that tent. youll definitely be able to grow some happy monsters in there. it looks like you dont have anything to reflect light in your attic so im thinking the grow tent is the way to go.