The PSone can be considerably easier to hack, specifically because you can simply rebuild the ISO with each major hack. For example, if one wants to replace a file with another of larger size, the task can be accomplished within a matter of moments. That said, CDmage, etc., is only useful for replacing files of the same size.
Things get hairy with PSone translation, however. You must either know or learn r3000a dis/assembly. In most cases, there's simply no way around it. However, once you've located the function that parses and displays the text, it becomes a breeze.
I say that dis/assembly is required, because the spacing between font characters - typically, Japanese characters are spaced further apart, as opposed to English. Of course, this isn't always the issue with every game in existence. You might get lucky, where only a simple texture hack is required.
I should also note that most developers used their own custom CD File loading methods. For example, most CAPCOM games use an internal structure (located in the executable) that details LBA and file size, which works especially well and more reliable than standard SDK functionality.
The best emulator for hacking is most likely pSX, as it includes a memory/RAM viewer/dumper, asm view, register viewer, VRAM viewer, etc.
All said, I do not suggest translation hacking, if you've never hacked on the PSone before. Start with simple texture hacks, and work your way up from there.