I thought I would write a little more in depth (no pun intended) on the sonar units. I am no expert by a long shot, but here is what I have learned that may help you in the purchase of one of the units.
1. Yes they are expensive. BUT...the pricing you see in Bass Pro and online, is simply the MAP. MAP = Manufacturer Advertised Price. Some companies will regulate the price that dealers are allowed to openly advertise their products at. But they do not regulate the price they can actually sell the product at. In other words I am telling you to find a shop or dealer that sells them, and do some shopping around with them. You can find them A LOT cheaper than the MAP. With big retailers you can't do that...what you see is what you get. So keep that in mind when shopping around.
2. Humminbird will be coming out with some really affordable SI units for 2015. They will feature an overall larger screen....but will still be 5 inch screen units. Right now, the current 5 inch screen models are measured diagonal. So they are not 5 inch screens from side to side.
3. The bigger the screen, the more you can see. Example...with the 5 inch screen units currently available...the transducer sees exactly the same thing as the larger units do. BUT...the information displayed on the screen will be really micro size. You can adjust the distance the side imaging is "seeing" off to the sides of the boat. The thing most people try to do, is set it for 80 or 100 feet off each side of the boat....thus covering a swath of 160-200 feet total. Now think about this...the unit will then try to fit 200 feet of info on that screen. You might go over a tree laying in the water that is 20 feet long...and on the screen, it might show up as being the size of a match head. If you can barely make out stuff that big...you don't have a chance at seeing smaller things like schools of crappie or bait fish. SO what you end up doing, is shrinking the overall scan distance. With the small screen units...to make the images appear large enough to see bigger stuff like brush piles, stumps, etc, you gotta go down to about 20 feet off each side...no bigger or they will be so small you can't see them. In reality...that's no better than a standard 2D image because it's basically what is under the boat.
The larger screen units will "blow up" the same image and make it appear larger. So now you can get away with going 80 to 100 feet off each side and still being able to identify stumps, brush, etc. The crappie are still hard to spot with the scan size being that broad. BUT what you are doing at this point is just scanning to find the cover/structure in the first place. Once you find that stuff, you can go back over it with a smaller scan size and REALLY see the details of what is there or not there, especially fish. If we are exploring a new area, we will usually scan with 100-150 feet until we spot something of interest and then we go back over it with smaller scanning. You'll usually see the "shadows" of a school of crappie or white bass before you see the white specks when scanning with a larger swath.
4. Humminbird vs Lowrance. We have Bird units. But having seen what the Lowrance models can do...I gotta say I think the Lowrance units have it going on right now. Their down imagine blows the Bird out of the water. Their side imaging is pretty great too. I can't afford one of those...the units are crazy priced and that doesn't include the side imaging transducer either. It's an extra $600 on those units...or it was. The newer Bird units are pretty easy to navigate around with. They do a really great job. We stayed on top of white bass today after they would quit breaking...knowing which side of the boat to cast to and let the spinners and spoons sink to, and continued to catch them.