@Jiryn I would go definitely for the PI and skip the Amiga Mini. Simply because the PI with RetroPie is far more flexible compared to the Amiga Mini.
Sure it looks nice but the diversity of emulation offered by RetroPie is something the Amiga Mini can't compete with. C64 games and Amiga games run perfectly on the PI. Haven't tried C128 yet byt my guess is it will run great as well.