There are threads on a couple of forums, including here, that have discussed antenna designs and construction. Some points that have come up there to keep in mind:
1) Mileage ratings are useless
. They arose to help translate a technical parameter (gain) to a more generally understood measure of performance. There is no technical basis for them, so manufacturers can say anything they want.
2) An antenna's size has to be a significant fraction of the wavelength of a signal for the antenna to be useful. There are a couple of designs, for example the Antennas Direct C5, that exploit a clever trick, but never expect a compact antenna design to be useful for channels 2-6.
3) You can't fight physics. Successful reception of ATSC television requires the signal to be a certain amount stronger than the background and man-made noise. The ONLY device you can get that increases signal-to-noise is an antenna. Amplifiers are useful in primarily to overcome signal losses in long
cable runs. There are a few special cases that take a while to describe, but in those situations it is easy to add a preamp later.
Thanks to the internet, it's much easier to size up the available broadcast TV signals to help figure out what antenna to buy. There are a couple of decent sites, but TVFool offers the most information. In general, one needs to pay attention to three columns in the table: Real channel (where the signal from transmitter to you is located), "NM" (noise margin, the strength of the signal relative to the minimum required for reception), and azimuth (to see how broad the region of sky is that the antenna must be sensitive to). More on how to read a TVFool prediction is here
Nick, to your situation. Note that you'll probably want to recieve everything down to WHYY to be sure you have all the major networks. That means you have channels in all three bands: VHF-LO (WPVI 6), VHF-HI (WHYY 12), and UHF (all the rest). The NMs are all positive, meaning, in theory, you don't need much antenna gain to make things work. However, to protect against signal fading and overcome the losses by receiving through the roof, you will need at least a moderate amount of antenna gain. An attic mount will likely work because all of the signals of interest come from one direction. My situation is very similar to yours, and I have an old medium-range Radio Shack all-channel antenna in the attic that works well. There is one proviso - if the antenna has to point out the side where the foil-backed insulated panels are, then all bets are off.
I think you are looking at a Winegard 7080P, which has elements that are sensitive to all three bands of channels. It's about $60 on-line.