Bob from RetroRGB did an XStation vs PSIO comparison video.
TLDW: XStation is twice as fast booting and loading a game for the first time but if you want to still use discs on the same PS1 then get a PSIO. If you're fine with not being able to play discs on that PS1, either because you have another PS1 or using a PS2 to play them, then get an XStation.