Attic installation comes up again and again. Roof mount is a lot more likely to succeed. Attic mount raises uncertainty... it could cut signal strength a lot, and introduce interference. Plus you'll want to go big with an attic antenna to overcome an shielding effects, and that can make mounting difficult.
I'm not an industry pro, so I'm just making a guess to get your thread going. If I wanted ABC (WPVI 6), I'd go with a maybe an Antennacraft HD1850, aimed at magnetic 188? Note this antenna has a 180" (15') boom. Attic mounting could be difficult. If it were mounted high above the roof, you could maybe go with a smaller antenna like a HD850?
http://www.antennacraft.net/Antennas...llChannel.html
Pretty sure you're going to need something with a long boom to pull in the longer wavelength signals of VHF.