My observation would be that since there are only a couple of makers for the "guts" that go into them, most microwaves are pretty comparable and interchangeable.

The thing to look out for is the proliferation and variaties of "presets" on the control panel. Many of the new ones don't really make it easy just to set a heat rate and time - you have to study up on whether "2 pouches of popcorn" is what you want to heat one "supersize soup mug" if the choices aren't translated into something that makes sense to you.

In the retail store, the operating manual is the thing you look at. After that, you can worry about whether it will look nice in your kitchen. Most of the rest is a matter of whether you get lucky, since makers swap factories more often than some 'catters probably change shorts.

John