Game structure. The MMO you mentioned is built directly on modified ROMs of official Pokémon games (eg FireRed, Emerald). It largely preserves the original storylines and mechanics but adds online multiplayer. PRO is a full custom fan MMO with its own maps, quests, and progression rather than strictly following the original games.
Gameplay mechanics. The MMO you mentioned closely follows official Pokémon mechanics (breeding, competitive systems, balanced PvP). PRO alters mechanics significantly (eg different spawn systems, no traditional breeding, grind focused progression).
Focus. The MMO you mentioned has strong focus on competitive PvP and an economy with trading tools like a Global Trade System. PRO is endgame grinding, also strong PvP focus, with dungeons, events, and cosmetic/event Pokémon.
Content style. The MMO you mentioned feels like playing official Pokémon games online with others. PRO feels more like a traditional MMO with custom systems, quests, and progression.
This is just my personal opinion. Asking about another MMO on the forum could be seen as advertising or against the rules, but I’ve tried to answer as fairly as I can if the question is genuine. I’ve played several Pokémon MMOs, including the one you mentioned, and I have respect for all fan made projects. In my opinion, Pokémon Revolution Online provides a deeper MMO style experience. Extensive custom content, more challenging progression, and unique systems that go beyond simply recreating the original games in an online format.