both are visible without precaution. in an outdoor shed, ambient heat is the issue, not hot spots. a good way to secure yourself is to line the entire grow chamber with 1/2-3/4in board insulation. i think tin roofs reflect infared (i may be mistaken on that)
most of the cases ive seen of an outbuilding bust has been detected from exhaust! simple solution is to vent out from the floor (there should be a space unless your building is on a slab)
but, cfl's should be safer, they produce less heat ultimately... i think you'll be fine with insulation, a cool tube for an hid, and a hidden exhaust.
let me know if you'd like help planning.