ECN brokers should offer you tighter spreads and an overall better trading experience.
In a way, your question is not valid as STP brokers no longer exist in many parts of the world due to regulatory changes. Your only real options when choosing a retail Forex broker are ECN or market market.
HIGH RISK WARNING: Foreign exchange trading carries a high level of risk that may not be suitable for all investors.
Leverage creates additional risk and loss exposure. Before you decide to trade foreign exchange, carefully consider your investment objectives, experience level, and risk tolerance.
You could lose some or all of your initial investment. Do not invest money that you cannot afford to lose. Educate yourself on the risks associated with foreign exchange trading, and seek advice from an independent financial or tax advisor if you have any questions.
Any data and information is provided 'as is' solely for informational purposes, and is not intended for trading purposes or advice.
Past performance is not indicative of future results.