One major difference with IB is that you can place an order between the spread or join the bid . You do not have to lift offer. In addition, the spread does vary and does get as tight as 1/2 pip. Thus your comparison is not exactly comparing apples to apples.
I use them both and no, they are about the same. Though IB's spread tightens earlier on average post news release. Anywhere from 2 to 3 secs earlier. Pre news, IB's spread remains tightened up a little longer than Oanda's on average by a few seconds also. Probably due to Oanda's "hedging" alogorithms vs IB's BBO. What I like most about IB is the various order types you can use when trading FX spot. It gives you more flexibility and consequently opens up a few more strategies. Oanda only has 4. BTW, your trade cost analysis wasn't bad. You should have accounted for IB's .5 which occurs by either playing in the spread or when the spread narrows to this which happens often enough. So you should have made IB's trade cost a range from $18 - $38 during "normal" market conditions.
He used 1.5 pips, I think that was on the generous side. IB spread average on EURUSD is more like 2 pips.
forget it, in volatile times 5 to15 pips slippage. The tip from IB: take limit orders, super, then you will not get filled!
So you are from Interactive Brokers, please be correct when you post. The commission for a roundtrade is two times 0,2 pip, plus an average of minimum 1,5 pip spread on EUR/USD is 1,9, plus you have to convert your profit loss by hand to your base currency and have to pay again a commission, incredible, just imagine you trade a few pairs!!! So Oanda for example is much cheaper, more convenient (your profit loss is on your account in base currency) and you don´t have slippage, whereas with IB you aren´t able to trade in volatile times.