i got a 14dbi directional horn for like $11 at microcenter on sale. hawkin - which is big POS. did the job well. google free antennas org - they have some neato 12dbi directionals for near $0 that do not require anything but paper and tinfoil.
omni's are okay but remember they fire like a mushroom down and out. which is why u want to keep an omni high high up in the air.
directionals just do what they say - narrow angle but precise pointing.
I've seen folks drop a hacked linksys to 1mbps and run 70+ miles with a home-grown directional antenna reliably.
some of the newer laptop's wifi cards have features for diagnosing signal that equal fluke meters that cost $5000. amazing.