If you hold overnight positions often, futures are definitely cheaper. The future price already takes into account the difference between interest rates in the "short" and "long" currencies in teh pair. However, the margin that a broker will charge (on small position) on top the interest rate difference will accumulate to a very substantial sum over the course of a year.
On the downside, futures may be a bit too large for a "small" trader. Say, EUR/USD CME futures (5E) are worth $12.5 per pip and a 100 pip (1 point) move, which in the current environment is very common in teh timeframe of 24-hours, means $1,250 change in the position value. It depends on the capital and risk appetite if the trader can afford to scale up this size of risk.
The liquididty in smaller futures (mini and micro) is pretty much no-existent. These are only suitable if the aim is to hold a position for a few days to a few weeks where 10 pip bid/offer spread becomes immaterial but overnight broker margin accumulates.
Aslo note that ECNs often have minimum commission on currency transactions. So, if at â¬125,000 full-size CME future size commission (per dollar size) is comparable, smaller currency transactions may incur substantially higher percentage commission.
For intraday trading it really depends on the trading style, during most liquid periods fractional-pip bid offer in spot currency may be attractive. However, at the same time broker ECN markets are usually more shallow than the futures. Hance, they are vulnerable to "running stops" - pushing the exchange rate a little further past a support/resitance level in expectation thsi will trigger stops. This stop running (as seen on the deviation of the move between ECN and futures) may range from a couple of pips to 10-50 pips (during important news). Hence, with spot currency one should be more careful about placing stop orders with a broker (as opposed to executing a stop manually).