I would avoid the attic antenna. Beyond the foil backed roof sheeting problem, you will find lots of home-grown RFI, both on the RX and TX side on the HF frequencies. The 20 ft "flagpole" vertical will work just fine. Just remember that the need for an abundant ground radial field is necessary for any sort of transmission efficiency. You can even elongate the vertical radiator by adding aluminum MIG wire (0.030" or 0.035") painted with a flat black or camouflage color. Use the MIG wire as either an extended top-hat or inverted-L. The wire is strong enough (I've had stealth antennas using the MIG wire and it only breaks on occasion - most likely from a bird flying into the wire. Easy enough to replace though. The painted MIG wire is virtually invisible - hard to see from ten feet away. Another option is to insert a multi-band vertical (Hustler, Hygain, etc) into a thin-walled PVC pipe. Finances permitting, the small Steppir vertical will suffice for the higher HF frequencies - add a loading coil for the lower frequencies.
BTW, the FCC OTARD regulations allow an outdoor "TV" antenna mounted on the roof inclusive of a mast extending 12 ft above the roof-line. A little creativity will open up lot's of stealth antenna possibilities.
A VHF/UHF stealth antenna is also easy. If you can extend a roof vent so that it's 19", you an tape a vertical to the outside. Lot's of other possibilities.
Scour to Internet to see what others have done.