Well it’s simple, the console obeys the game meaning if you use a NTSC game on a PAL system (with chip) you get NTSC standard out, that’s why if you only use composite most old TV’s gave you black and white picture due to not correctly handling the standard. If you use RGB, which you should you get color as it’s covered in the standard. The RGB signal will be 60hz using a NTSC game and it will play exactly as it would on a NTSC console, also PAL used more lines which in the case of OSSC can be a problem.
For ex. I have a PAL PSX system chiped with an arduino;
When I play metal slug X I get 60hz or 59-60hz in 263p meaning when using OSSC with lineX2 I get 60FPS 526p out which is no problem for the TV.
If I play my PAL version of Battle Arena Toshinden 4 I get 50hz or 49-50hz but also a resolution of 314p, this game does not play well with the OSSC hdmi output, the OSSC sees it no problem but the output is outside standard tolerance for my Sony TV as it’s either 628p with lineX2 or 314p using pass-through, have to try and fix that someday.
Same goes for interlaced material so you can get either system, PAL games will get you a higher resolution and more graphic fidelity at the cost of frame rate, but for some games that will preferable while others you prefer the speed at a loss of some picture quality.
So take what ever is cheapest and go with that, some PS1, PSX, PSone units are said to be better than others but honestly the difference it minimal.