Hi Bup...there is so much to choose from out there that I think you need to give a budget. For some people on this board "no breaking the bank" is $100 - for others it's $1000. Try to put a price tag on what you can spend and you will get some more accurate recommendations.
I upgraded a "lower price range" unit this year to a mid to upper price range unit and the difference is HUGE. I have a combo unit and both the GPS/mapping features and sonar are night and day from the unit only a few hundred dollars less. I'm now seeing bait, my DR balls, all fish (even fish rising in the water column) and can sound bottom even at higher cruising speeds. I'm sure I will also see the thermocline once it sets up which is huge when summer kicks in.