I just went for the cheap (like seventy bucks) Eagle/Lowrance unit.  I think it's called a Cuda 300.  The thing is not fancy, but it has worked flawlessly.  And boy I've beaten the tar out of it--having been flipped in the surf on more than a few occasions.

It ain't fancy though.

My friend just bought a Lowrance unit that has the  GPS-FF, and I got to say it's pretty awesome.  It shows the bathymetric contours with your track on one side (a chart and your position), and the Sonar images on the other.

If I can ever get this one to die, I'm going for one of those.

I've owned both Humminbird & Lowrance & they both will get the job done. Since your primary fishing is offshore trolling I would strongly recommend getting a combo FF/GPS unit. Not only will the GPS get you straight back to the launch when the fog rolls in it really helps to mark a waypoint when you get a bite that doesn't stick so you can use the GPS to swing back around and troll exactly back through your path to try & get a second hook up. & a color screen is way easier to read & differentiate between bait, fish, kelp, rock bottom, sand bottom, etc. I've been very happy with my Lowrance Elite4 for $300. A standard battery will last all day & then some.

I guess the one thing I would look for is to see who has a plug to cover the ends when not in use.  This is something that has annoyed me that I have seen some of the makers don't make a plug to go over their connectors.

Humminbird makes a cover for the power head.

I looked at a few at WM, the one that caught my eye was the Raymarine Dragonfly, but then so did the price tag

I remember SBD posting a positive review, anybody else used it?

I am pretty happy with my Raymarine Dragonfly 5.  Check out the video below.  The screen is amazingly bright and easy to read even in full daylight or heavy glare conditions.

All the new Lowrance HDI units should come with caps for the plug ends. Also you can use the same ducer and power plug for the 4, 5 and 7. So if you do decide to upgrade to a bigger unit, no need to rerun any wires, head unit upgrade only and the other head unit as a spare/sell.

Mike