To get better information, you need to specify more of the model number .... Some units are 2D only, some are 2D and DI, and some are 2D, SI, DI.
I will assume you are asking about the SI series ......The 8-1100 series are all going to operate pretty much the same except the screen size .... but the 899 comes standard with a compact SI transducer. Humminbird will exchange the compact transducer (for an upgrade fee) if you contact them before opening the packaging on it.
You might take a look the the Onix units also .... as some say, they are newer technology and more "future proof." The NT (which don't have touch screens) series will be more affordable.