I did a comparatively extensive comparison between FANZA and DLsite image quality. Search the forum for topic 'FANZA vs DLsite image quality comparison'.
Summary: There is no way for me to truly know which one of the two displays original untouched files from the publisher. However, there is no distinguishable difference in "QUALITY" between both services despite the file size differences. Manga that has twice the file size on DLsite looks absolutely the same as the FANZA counterpart. The only thing I noticed is that sometimes FANZA has darker shades than DLsite. Oh and resolution topples all. So that should be the determining factor.
I buy almost everythijng on FANZA. Why? A decent freakin mobile reader app FANZA Books. DLsite has an abomination mobile app or a web reader. But it's 2025 and I'd rather have a dedicated app for my tablet. And FANZA has much more exclusives and, you're not gonna believe it, better censorship for quite a few tankoubons earlier than 2015.
I usually purchase products from Fanza, DL, Melon, and several others. I've come across instances where there is a clear difference in material quality, especially with older titles. But you're right—lately, the differences have become less noticeable. Still, I wanted to ask for an external opinion.
Unlike you, I don't use the apps; instead, I rip the content and use my own readers (such as Komga or Kavita). In this ripping process, I've encountered situations where both Fanza and DL publish content in truly terrible quality. I've even had to repurchase the title from another store just to get a decent version.
I have almost 10,000 tankoubons on FANZA and 500 on DLsite. Ripping is absolutely not an option for me with those numbers, so my main factor is ecosystem and convenience, then quality and the last price.
I have no access to Melon as it requires Japanese phone number.