If all the transmitter sites are within a 40 degree arc, then directional is your best choice, and will provide slightly better signal strength.

While an omni-directional antenna would receive them as well, it could pose problems in areas with foothill mountains or tall city buildings behind the antenna.

That is what is traditionally is known as multi-path distortion or ghosts in analog broadcasts. The transmitted signal is reflected from behind and slightly delayed when it reaches the antenna. While digital broadcasts are more capable of dealing with this, they are not immune from problems.

As far as a metal roof, as long as your antenna is above the roof line, there should not be any issues. Which just occurred to me, if you were considering an attic mount, a metal roof makes this a no-go.

Also make sure your ground the coax, through a coax grounding block, to eliminate static buildup which can cause "sparklies" in the picture or can occasionally, if the static build-up is bad enough, even cause tuner failure.

The mast itself should also be earth grounded for lightning protection - 10 gauge wire or larger.

Thanks, very much. I don't know how much potential there is for multipath in my neighborhood. There is a relatively huge bridge nearby that is quite high. I don't know if it would act more like a reflector or a Faraday cage.

For sure going for the roof-top mount metal roof or not. Can you say what size arc would put me in the omni category? I might find out that my tower observations are not correct, though I am reasonably convinced after looking at antennaweb.

The easiest way to compare antenna coverage is to look at the polar plots and beamwidth specs published by better antenna manufacturers.

Here's a link to Winegard's television antennas. If you click on the spec sheets, it'll provide you with the information.

Here is a link that might be useful: Winegard Antennas

It's almost impossible to make a good recommendation without knowing the lay of the land: I find that often people's assumptions about their reception conditions lead them astray. Try this go to TVFool.com, enter your local information (it remains completely private, and won't reveal your location to anyone) then post the results here. With that information in hand, we'll know what issues you'll face when installing an antenna, and better know what kind of antenna you'll need.

Here is a link that might be useful: TVFool

Thanks, that is nice information to have in addition to antennaweb.org.