currency futures is a regulated market, spot is not.
with currency futures you are trading directly in the futures market (unless its a false synthetic garbage indicative price set by a bucket shop).
FXCM & Onanda are both bucket shops and your order is with the broker on "their indicative price" which is not the "spot interbank price").
currency futures typically have a better bid/ask spread than the spot interbank market. You pay a commission (which adds a virtual 0.5pip to the bid/ask), however it still usually works at cheaper even with a market order.
re:markets orders, in a futures market you are guareenteed a fill in a volitile/fast moving market. FXCM may lock you out completly (even if you want to exit a losing position), Oanda will just increase the spread massively even before the event because thier price feed lags the real market, and even though the real market will have wider spreads during this time, Oanda will leave the widened spread for longer than "normal". That said i have also noticed that the futures market can be even more erratic (and risky) during major news events than the spot market.
want to sell the "high" & "buy" the low? well in currency futures you can sell the ask & buy the bid (and pocket the spread, just like a market maker). you dont have this option with FXCM or onanda, since you always pay their them their spread. I only know one "retail broker" that offers this option for trading spot FX (as well as futures).