We have the Simrad AC42/AP24 and also had your question when researching them.
The difference between the AP24 and AP28 is that the AP28 has a steering
knob, a larger display, and the data on one of its pages has a graphic - where on the AP24 those data are presented as numbers.
In other words, they have identical functionality, except that manual steering
is done by a knob on the AP28 and a button on the AP24, and they present identical data, with the exception of a single
graphic on the AP28 instead of the numbers on the AP24.
Both displays are multifunction devices that display all data on the network using multiple user-customizable pages in addition to their primary AP functions. Neither are good looking MFD displays compared to the newer small dedicated ones like the B&G Triton or Raymarine
Our AC42 controls a Raymarine linear electric
We love the system and have had no problems with it.