any yagi beam antenna will focus your emissions with most of your signal going twords the major lobe (front) and the remaining part of your signal being radiated 180 degrees away (back) with none being transmitted to either side. this focusing helps keep go farther. it is what they consider a power multiplication factor. its like you ar eputting out more power because our signal is focused. you are noty actually adding any addition power but the antenna works like it does. when my buddy rick put up a 104c verticle 65 feet up we tested it vs is imax 2000 omni, switching between the 2 antennas showed a 3 s-unit signal from the imax vs a 7 s-unit for the 4 element beam.
if you are talking in town an omni antenna is a better choise. it does not reject signals from the sides and rear unlike a beam will. if you like aways away from town or most of your signals will be coming from 1 direction a beam is a good idea.
there is no advantage to having a beam verticle or horizontal, what i mean is they work equelly well both ways. the main difference is this. if you are wanting a beam to talk to your friends that are mobile or have base antennas that are verticle than you should install your beam in the verticle position.
if you are wanted to get a beam for a shooting skip than you will want to mount is horizontal as most skippers with beams are mounted this way.
there is a large signal loss when the transmitters signal is horizontal and the recivers is verticle (or vice versa) that is why you need to decide what your main focus will be.
the big advantage with the hv series beams is that you can switch between verticle or horizontal, so when you are rag chewing with your buddys you can run it verticle, but when you want to call cq you can hit the antenna switch and make the antenna horizontal.
the disadvantage to a beam is that you need to buy a rotator to spin it if you do not want to go do it by hand every time. they are only really effective at recieving signals from 1 dirrection. where ever you antenna is points is the people your going to hear. this is whay many people will use an omni directional antenna as well. the transmit in all dirrections ok and recieve from all directions great.