[GeoffR]

For novels I prefer to buy the individual ebooks, but with some older books an omnibus is the only available option. For shorter works I prefer single-author collections.

I might buy an omnibus instead of the individual ebooks if the omnibus is a lot cheaper, but I am a bit wary because the omnibus version is often just a crude merging of individual books* with incompatible styles rather than a properly constructed ebook in its own right.

* Edit: This can make it simple to split them up into individual books again though.