For NLHE/PLO50 and below the Microgaming Network offers the best conditions! While you're learning the game it's best to play under the best side conditions to increase your winrate as much as possible. These extra benefits will soften your downswings that are inevitable, and the impact is thereby diminished.
The Microgaming Network offers the best rake structure for these stakes compared to other sites. Next to that, imagine the difference a welcome bonus makes that clears at a 20% rate, 30% flat lifetime rakeback and a rake chase that can be offered.
Check this comparison for example between Pokerstars and Betsafe (MPN) for PLO25. Note that Pokerstars even has a decent rake structure for these stakes but the lack of rakeback and welcome/reload bonuses are a disadvantage.
888Poker effectively charges 5% uncapped rake per pot since the rake caps are never met, plus there is no cash rakeback.
ps. For PLO, there aren't any other fast-fold formats running besides Pokerstars.