My thought is that you are comparing an apple with an orange.
The 984 family is now a quite old Modicon product, of a similar age to AB's PLC5, and was a similar match in functionality.
By contrast the MicroLogix 1500 was released in the mid 90''s as a small low cost expandable "OEM brick".
Although the Micro 1500 is quite useful, it offers limited comms options ( 2 serial ports, and a DeviceNet scanner), only two memory sizes, limited max I/O (about 16 modules I think), and no online editing. Now this may be just fine for a smaller application, but if you are comparing with a mid-large sized application that the 984 could tackle, then the limits of the Micro will be a show stopper.
A fairer comparison would be between a Modicon 984 and the larger SLC500 processors (5/05,4, or 3) or indeed I would be seriously considering either ControlLogix or a CompactLogix depending on the size and nature of the application.
And to be fair Modicon offer newer systems than the 984. So all up I too am curious why you are making the comparsion between just these two PLC's.