fish finder /chartplotter

( I moved this post to proper location)
I am newbie when it comes to using technology.
Just purchased a 17 foot duroboat with two downriggers, and now wanting to pick a fish finder/charplotter combination,

Too many to choose from:
any advice:
I saw one at fisheries seattle: garmin 440S model with gps and fish finder(sounds to be good model on reading the specs)

question I have and did not really understand is what is the difference between dual beam technology versus duel frequency??

any other models you would reccommend which has both charplotter and fish finder in one unit,

mounting the transuducer will be on the transom of the boat
thanks alot