Just got a GE Profile microwave (2.0 CF) to replace my dead Jenn Air. Do I get the 27 or 30 inch trim kit? The cutout width is 28.25; the cabinet above is 28 and the oven below it is 30 wide.

You will need to go with the 27" because of the cutout size.  According to the instructions of a trim kit, you will have to install other things in the space before you actually get the microwave in , so you really need that bit of extra space, not only for that, but a microwave in a cutout like that needs a bit of room for venting!  Here is the instruction manual for you to take a look at!
http://www.specsserver.com.PDF