Unlike the Audya 4, 5, 8 and 76 key (which are all identical with the exception of some hardware differences such as keys) there are differences in the way the MidJay Pro and SD7 operate...and there are also many similarities too. The two instruments are aimed at different users - The Midjay is for performers, vocalists, K-Jays and multi media entertainers but can be used as an arranger module, The SD7 is primarily for keyboard/arranger players but can be used for K-Jay, backing and general multimedia entertainment.
The Operating System of the SD7 has been adjusted to suit an arranger keyboard and this is the most obvious difference when using either product.
This is also evident in other areas with carefull thought to which hardware options (buttons/sliders) are located on the top panel and which are on in the various on screen menu. The SD7 has the all important hardware arranger buttons below the screen whilst these are located on the arranger screen of the MidJay Pro (in my oppinion actual buttons are vital for an arranger keyboard).
I have not compared sounds and styles, but there are definately different styles on the SD7, wether these will be added to the Midjay Pro it remains to be seen.
The SD7 has not been in long enough to tell you any more - but if anything obvious comes up will let you know.