• Wii, you’d be perfectly fine either way, you’ll be able to get 480p 2x on the OSSC or 3x on the 5X.
• PS2, you’d really benefit from the 5X for a vast majority of games, which output 480i, as the deinterlacing on the 5X is much better than the OSSC, here’s an example on GT4, and you’d again get 3x (1440p) vs 2x (960p) on the OSSC.
  You could try and force 480p via GSM otherwise, if your PS2 is nodded, but I’ve had pretty poor luck with that method so I wouldn’t recommend it.
• PS1, that’s debatable, for most games you’d be completely fine either way, but there’s a fair number of them with res switching, which the 5X tends to handle better / without any delay vs the OSSC, so if that matters to you, that could be another pro for the 5X.

You’re kinda in the perfect “target demographic” for the 5X with a PS2 in a way, but it’s a matter of how much flickering bothers you or not, and whether or not the higher output res, more features, etc, is worth the price or not.
Again, both options are absolutely great and you wouldn’t go wrong with either!