I usually trade on BMF - Brazil, but I wish to expand.

I was considering Oanda or Alpari x Mirus ou Interactive Brokers.

Which is cheaper for futures?

Alpari and Oanda charge the SPREADS, but I don`t know how much it is the slippages on IB and Mirus plus the fixed costs. I notices that many users on big mike are on IB or Mirus.

Alpari future spreads can be found here:

MetaTrader 4 Commodity CFD Product And Contract Specs | Alpari UK
MetaTrader 4 Index CFD Product And Contract Specs | Alpari UK

Oanda futures spreads can be found here:

Spreads ao vivo | OANDA fxTrade

Alpari is offering CFD, not futures. IB is offering CFD and futures, Mirus futures only.