IB forex commission works out to 0.4 pips per RT (round trip) for trades over $125,000.
For any forex trade from $25,000 (the minimum allowed size) to $125,000, IB commission is a a flat $5.00 / RT. For the minimum $25,000 trade, that works out to 2 pips.
Therefore, for EUR/USD:
over $125,000 --> TEC (total effective cost) is 1.4 pips at IB and 1.5 pips at Oanda;
$25,000 - $125,000 --> TEC is from 1.4 to 3.0 at IB and 1.5 at Oanda;
under $25,000 --> TEC still 1.5 at Oanda, IB IDEAL Pro not an option; IB IDEAL has much wider spreads; CME currency futures on IB @$4.22 promotional rate; EUREX currency futures less for now.
In particular, for a 100K forex trade, it's a dead heat at 1.5 pips each, not "a better result".
However, IB EUR/USD spread is variable, not a fixed 1 pip, can be 2 or more at times. Oanda EUR/USD spread is fixed 1.5 pip, but can increase to anywhere from 1.8 to 3 pips for a few minutes around scheduled news release times and 10 on weekends (when IB is closed).
Having said all that, a number of key factors other than commissions ought to be considered as far more important for 99%+ of traders.
