Kindle Previewer 3 versus KDP Online Previewer

Hi

I am self publishing a cookbook on KDP. It is a fixed layout ePub format.

I verified this book on ePub verifier and there were no errors. It also looks perfect on Kindle Previewer 3. However when I uploaded the manuscript on KDP and previewed it on the 'Online Previewer' during the uploading process, it looks wonky with numerous formatting errors, such as out of whack character spacing and paragraph styles running amok.

My question is ... which (Previewer 3 versus the 'Online Previewer' used during manuscript upload) is the more accurate to use for the purpose of testing how my ebook appears on Kindle readers?

Thanks!