Choosing a Forex Broker

Market-Makers
Pros:
Usually give free charting software and news feed
Prices can be "smoother" and less volatile than ECN prices (this can be a con if you are scalping or trading very short term)
Often have a more user-friendly trading and analysis interface
Cons:
They may trade against you. In that case, there will be a conflict of interest between you and them
The price they offer you may be worse than what you could get on an ECN
It is possible that they may trigger stops or not let your trade reach your profit target levels by manipulating prices
During news, there will usually be a large amount of slippage; their systems may also lock up or not allow order placing during times of high volatility
Many of them discourage scalping and put scalpers on "manual execution" which means their orders may not get filled at the price they want
ECNs
Pros:
You can usually get better bid/ask prices since they come from several sources
Variable spreads between bid and ask may give no spread or tiny spreads at times
If they are a true ECN, they will not be trading against you but will pass on your orders to a bank or another customer on the other end of the transaction.
You will be able to offer a price between the bid and ask with a chance of it getting filled
If they support Stop-Limit orders, you can prevent slippage during news by making sure that your order either gets filled at the price you want or not at all
Prices may be more volatile which will be better for scalping
Cons:
Many do not offer integrated charting
Many do not offer integrated news
Many of the trading platforms are less user-friendly
Because of variable spreads (between bid and ask,) it may be more difficult to calculate stop loss and profit target in pips beforehand.


Read the whole article:
http://www.goforex.net/market-makers-ecns.htm