As the son of a TV serviceman (anyone remember those guys?) and antenna installer, I recall him teaching me there are three primary criteria with antennas for TV and FM.
One was pointing towards the source of the broadcast antennas. These were directional antenna that typically have 3-5 arms making them more capable of receiving weaker signals and provides a stronger output to your tuner. An electric rotator can better point to a variety of broadcast sources. As many have stated, omnidirectional antenna receive signals equally from all directions but are usually a single whip metal rod yielding a weaker signal.
Two was that TV signals were primarily "line-of-sight" or straight line signals making antenna height directly related to distance from the signal--further away means mount it higher. Some were even mounted on home towers 75-100 feet tall.
Three is outside mounting is MUCH better than inside. Walls and roofs block signals and weaken their strength.
Hopefully this helps you decide which to choose for your situation and narrow the trial and error process.. Generally, if you're within 50-75 miles of the FM broadcast antennas you want to listen to, you'll likely find many options.