Lowrance put out a very good book on sounders awhile ago, called the
Lowrance book of Sonar & GPS was written by Ron Calcutt. It does talk a lot about Lowrance sounders but overall the information applies to all sounders.
Recommend you try get a copy of this book or later editions as it
covers sounders very well and also to how to read them, and make sense of all the data they can supply.
The quick answer to all your questions is: It depends where you are going to use the Sounder.
Generally a 20deg cone angle @ ~192-200kHz is a good compromise for most average users and is ideal for most fresh water and salt water bay, estuary and inshore fishing. A sounder with a narrower cone angle and similiar frequency (8deg @ ~192-200kHz) is good for ocean depths up to about 500ft. To users who want maximum depth capability or for Ultimate depth capability use a lower frequency eg 50kHz.
There are probably 5 main elements that determine the performance, depth and degree of information you can see on your sounder screen
The Software within the sounder that analyses and intreprets the returning sounder signal
The Frequency of the transducer
The cone angle
Screen Resolution: Basically more pixels you have the better. Anything below 100pixels Vertical will not have enough resolution for seeing great detail. The screen width Horizontal Pixels shows the history of the signal thus is not as important.
Output Power: The more power = more sounding depth
Cone angle is generally related to how deep you want to use the sounder (ability to show the bottom not an ability to clearly show fish) eg. think of a torch light with an adjustable beam shining against a flat wall, with the adjustment set to wide beam you can see a great deal of the wall,
thus in the case of a sounder a large cone angle will mean you can
see a lot at a shallower depth, whereas if you adjust the torch beam
into a narrower focused beam you only see a smaller but more intense
area of the wall but can do this at a further distance, similiar a
smaller cone angle will mean you can use the sounder at a more focused
but deeper depth.
As the bottom gets deeper the sounders bottom coverage widens. eg if you move away from the wall with the torch the spread of light increases.
This should mean a 20deg transducer would give massive bottom coverage as a boat move into deeper water however this is not really true as the sounders starts to loose edges of the signal and bottom coverage. A narrower cone (8deg) transducer would in practice give a similiar bottom coverage at this stage however will work to deeper depth.
Low frequency tranducers have excellent penetrating ability, although a lot of the edges of the signal are lost, the centre cone has excellent sounding depth. They are very good for determining the nature of the bottom.
High Frequency transducers are superior in delivering detail. They can seperate small bait fish and show them as individual targets, low frequency units will lump the object together.
Dual Beam / Frequency refers pretty much to the same thing, there are two frequencies or cone angles available in a given transducer. Some sounders have the ability to combine both sounder returns on the screen to give the best of both worlds (Depth and definition).
Hopefully this hasn't confused you :?
I've been looking for a sounder for my Yak, however I've been tossing up mainly over price and where I intend to use the sounder. My choice at the moment is the Humminbird Matrix 12 / 17. However for a $100 less I could get a small Eagle sounder which would probably work just as well. I havn't mentioned Lowrance even though they do make very good sounders as the Eagle units are made by the same factory and are cheaper. The main reason I like the Humminbird is the fact I already have an older Humminbird on my boat so from what I can work out I can interchange the sounder just by pluging it into the base mounted on my boat. The Matrix also has inputs for a GPS which could be useful... Then again I just love toys