For the most part, there is only one big player for the manufacturer of transducers for recreational boaters and that is Airmar. Most manufacturers do not make their own transducers. They write their own specs, and then put out a contract to Airmar. Many of the transducers used by one company are nearly identical to those used by another company. As most boaters have figure out, they are not interchangeable by design. The spec for the connectors used by manufacturers varies all over the place, hence even if the wet end is identical you would still not be able to interchange transducers. Manufacturers could standardized connectors, but they won't for obvious reasons. This does not mean you cannot buy the correct Airmar transducer for your boat since they know what spec each manufacturer uses for its connectors since Airmar was likely their supplier. If you go to the following web site:
AIRMAR MARINE PRODUCTS
you can see a neat guide for just about anyone interested in the business of obtaining the correct xducer for their ride. Their web site also has cross references for just about every manufacturer's machine to Airmar's part number. Since only you know exactly what you will be doing with your fish finder, there is no pat answer as to which one to buy. If one is just looking for submerged rock piles for tog fishing, the low end of Hummingbird's units will suit you fine. If you are commercial tuna fisherman, who wants to know what is going on for miles around you, and have to worry about snagging your net on your high resolution sensor array as you pass over the net, then you might be interested in a high definition unit by Westmar with its retractable hydraulic sensor at a price that would exceed the cost of most people's ride. The unit I read the specs for was able to pick up a Basking Shark, on the surface, two miles away. If someone recommends a unit to you, make sure he is doing the same same thing that you plan on doing and if so, then go for it.