Transaction cost in fx futures have never been cheaper than cash fx. At least not with reputable fx brokers that only pass through liquidity and charge commission without any markups or alteration of the price feeds.
Carry is built into fx futures so there is zero benefit in that regards of trading fx futures. My analysis has shown, and I trade around 400mln-1bln in usd notional per month in cash fx, that for me overall cost of execution are lower via cash than futures. But then again the cost comparison depends on a lot of different variables such as how specifically you execute in size should you trade larger volume.
In fact you may be exposed to additional risk trading the futures each time you trade the rolls.
In addition, only a few fx pairs are offered via futures whereas in cash you can trade close to 120 pairs.
With all due respect, this is mostly incorrect. Which FX broker do you work for?
The wider choice of pairs is true, but why confine yourself to a single asset class in futures? I'd rather trade 40 futures markets across different asset classes than 40 FX pairs that are mostly going to be pretty correlated.
Execution costs are pretty similar in futures and FX, except perhaps in massive size (non retail, if you have RFQ access to multiple banks.... maybe that is you....<shrugs>). The cost of rolling futures then is doing 4 trades a year - peanuts. And there is no risk if you do a spread trade (also usually cheaper). Futures also have much lower counter party risk. Even if you're an institution I'd rather be exposed to a clearing house than a gilt edged prime broker; and most retail customers are going to be exposed to some pretty dodgy broker credit.
"Carry is built into fx futures so there is zero benefit in that regards of trading fx futures"... is the biggest misleading comment. Probably the main benefit of futures over FX is the cost of carry.
Let's take USDGBP as an example. From the IB website the reference rate for USD Is 2.19% and for GBP is 0.801%. So you'd earn about 1.39% one way round and pay it the other way round on a futures trade (I say 'about' because the futures are priced off forward interest rates and a slightly different credit - but you'd still earn X and pay X).
However on a spot trade IB will only pay me 1.69% (above $10K) on USD and they charge me 1.801% to borrow GBP (if I borrow more than 80K). So instead of earning 1.39% carry I'm paying 0.11%: 1.5% difference. Doing the trade the other way round IB charge me 3.19% to borrow USD, and only pay me 0.301% in GBP interest. So instead of paying 1.39% carry I'm paying 2.89%, again 1.5% difference.
The 1.5% financing spreads for IB are actually quite competitive, most bucket shops charge 2, 2.5% or even more. Even institutional funds have to pay financing spreads - which is why if they have any sense they buy FX forwards rather than doing spot trades.
Because FX vol is quite low paying 1.5% a year in carry costs is an absolute killer. I'd agree with the other poster who said you shouldn't trade spot FX unless you're closing your positions every day (which I also think is insane but that's another story).
GAT