I use PBP files simply as its the "industry standard" for compressed PSX games, as thats what they were converted to to play as eboot games on the PSP.

Chd files are great too though.

I have every MegaCD game ever released in chd format, along with the PC Engine cd games, and have all the NeoGeoCD games in chd fornat, as Recalbox 7.2 will load these 👍🏻