Ironically enough I was contacted by another member here late last week about selecting a sonar unit. As I shared with him budget and how you plan to use the unit pretty much dictate your options. Appended below is what I shared with him based on a budget of $150.
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At that price point your options, that wouldn't be just junk, are going to be fairly limited. You'll certainly be looking at smaller b&w units without any GPS or mapping functions. Not that there's anything wrong with that, heck like anything electronic the technologies change so fast that even the lower end units are better than what the higher end units were not so long ago. There are some color units in that price range, but I wouldn't suggest going that direction. Basically in order to get the color screen you'd be giving up a lot of things that I think are more useful on a practical level.
Anyway, the main characteristics I think you want to look for at any price point are the power output and resolution. In my opinion probably the best way to get a sense of how one unit's resolution compares to the other is to compare the pixel density for whatever units you're looking at...and more specifically the vertical pixel density. So for example if you were looking at two units that have the same screen size, and one says it has 240 vertical pixels while the other has 320, all things being equal the latter should give you better resolution which ultimately translates to better target definition and separation.
However, having said that, power output is also important, at least as important as pixel density. The lower the power output the less likely the sonar is going to detect a target at whatever depth and the less well it is going to define it. Personally I'd always opt for a higher power output in any unit I was looking at.
A couple of other things to consider are screen size and the number of grayscale levels. At whatever price point one looks at, for any given vertical pixel density and power output I'd always opt for the widest screen in the options I have. For me, and I think most people, a wider screen simply makes it easier to understand what you're looking at because it provides more context, so to speak. Also, within the b&w units the higher the number of grayscale levels the better off you'll be in my opinion.
I've personally owned Lowrance (currently do), Eagle, Hummingbird and BottomLine sonars. I've also fished on a lot of other folks boats with Lowrance units on them. Of the units I've personally been around and used my favorite has been the Lowrance units. I'd absolutely stay away from the H-bird units in this price range. I do know of some folks that have some of the lower end Garmin units but I don't know anything about them first hand, so I'm reluctant to say anything one way or the other about them. Of these, I'd suggest you take a look at some of the Eagle units. Maybe the Fishmark 320 or 480 units. The Eagle units are basically dressed down Lowrance units or utilize technologies that were in the Lowrance units a couple of years ago. Which for your use probably wouldn't make any real difference. They are made by the same company, just a different name and price. Sort of a Ford vs. Lincoln kind of thing.
As for where to buy the unit, the best place I've found is By Owner Electronics...here's a link to their website:
http://www.boemarine.com/You might consider calling them and talking with Jim. I've found him to be knowledgeable and helpful.
Then too, around this time of year sometimes you can find a good deal on a unit at Cabela's. I've had very good service from Cabelas and they are my overall favorite store for fishing/boating related stuff. Excellent customer service in my experience.
Lastly, you might consider searching through E-bay. I'd suggest staying away from individuals selling a used unit there and only consider dealing with a seller that has many, many positive reviews and appear to also have a brick and mortar store somewhere. These gizmos, like almost all electronics have become a commodity item and in today's economy at this time of year I suspect you might find some smaller stores trying to clear out inventory so yo might find a deal that way. But be careful about who you deal with!
Good luck with whatever unit you decide to go with. If you get it narrowed down to a few units and would like an opinion shoot me a message and I'll have a look. Once you get it on the boat, I'd encourage you to turn off any "Pro/Auto" mode type settings and learn to adjust the units settings manually. The little fish ID things are not a good idea either. Finally, installation of the transducer is genuinely critical to making any unit function properly. Slight differences in the height of the transducer on the transom, where it's at relative to any strakes on the bottom of your boat as well as it's angle relative to the water's surface can make a bid difference in your results.
Good luck,
Tim
