You often want to have a background of white in a laser cut image that will represent the top of the material. Then the last one is 16-bit Tiff which will give you a much smoother set of grays (and is the best option if trying to use this for outputting a 3D grayscale map for other software) - not sure if there are lasers supporting this option but if they do in theory it would give you the best results. There are 5 output options - jpeg, bitmap, tiff and png which I believe will each give you 256 levels of gray to represent the Z range.
FYI if your looking to test any lasers that have this functionality you can export the composite 3D model from Aspire by using the Model drop down menu from the top of the interface and picking 'Export as Greyscale Bitmap'.