Apologies for the late reply - I've been traveling a whole lot the past couple of weeks...
It appears XISF is a PixInsight-specific format (though open and royalty/patent free), however PI will keep supporting FITS (as it should).
As for the practical difference between Integer and Floating Point FITS, there are some small, subtle differences, which have to do with the way floating point numbers can encode data that aren't really numbers (such as Not-a-Number or +/- infinity).StarTools includes some extra routines to interpret such non-numerical values when importing floating point data, so that such missing samples get converted into 'best guesses' in order not to 'upset' the many algorithms in StarTools that require an artifact-free image as input.
Given that StarTools' internal engine uses 64-bit integer arithmetic, it is also theoretically possible (if you're feeling particularly evil) to specify a dynamic range that cannot be sufficiently encoded using 64-bit integers, causing banding.
Long story short, you will probably notice very little between FP or integer encoding, but Integer-saved FITS will make StarTools' job a lot easier (and loading times possibly shorter).
StarTools creator and astronomy enthusiast