As regards image quality, Mobi (and apparently KF8, if I have read the documentation and the release notes correctly) have a decided limitation of a maximum filesize for a source image of 256kb. Anything above that it will downconvert.
This may be subject to change in the future, as it until recently it was just 128kb, and a mere 64kb about a year before that, so Amazon may potentially make more changes to allow for larger image sizes in the future, but it's something to keep in mind if image quality is important. Sometimes the downconversion leads to some nasty pixellated blurring on maps included with sf/fantasy books, I have found.
ePub does not limit your image size and you can put in a very large original, though any given reader may not be able to take advantage of such high resolution and may choke on a large ebook filesize as well. But it's there if you want to use the option.
Also, Mobi is a compiled file format and you can convert to and from Mobi, but you can't open it up and edit it directly like you can with ePub, which is basically just regular HTML and some other files in a slightly fancy standard zip archive. Mind you, Mobi is based somewhat on the older HTML version, so once it's unpacked you can edit it readily enough before recompiling, but it will generally only understand a limited amount of HTML formatting features and any CSS must be "translated" into the kind of HTML that Mobi recognizes, which can result in very plain (or sometimes messed up) formatting which admittedly most people would never notice (unless it was really messed up).
You may well end up losing stuff going back and forth, but there are some newer Mobi handling tools which can be found in our Mobi format forum will hopefully keep such losses to a minimum if you choose to go that route (easier to just maintain an ePub source and convert to Mobi as needed, IMHO).
Hope this helps.