Imo, "ECN" refers to the type of exchange or market structure on which a security is traded. "STP" refers to how a security is traded, specifically how orders are routed and executed and who are the counterparties to your order and etc. The two concepts are related.
ECN brokers offer more speed and transparency but demand substantially higher commission fees. STP brokers have inconsistent processing times and reduced order routing transparency but generally offer favorable pricing.
I like trading on an ECN account because of the instant execution of orders at my broker fxopen, as well as minimal spreads (for EURUSD 0.01-0.02). This is very profitable and comfortable when you are engaged in intraday trading, where transactions even with a small number of points give the opportunity to earn.
ECN is a network of dealers which broker uses to provide best available price in the real time and can participate and act as a dealer too. STP means brokers sends your order up into the stream and never participates as liquidity provider. ECN is often cheaper because best available price means spreads will be lower on average.
Alright, here’s the deal: most traders prefer ECN accounts because they give you direct access to the market, which means you get better spreads and faster execution. ECN accounts connect you straight to other traders or big market players, making trades smoother and more transparent. STP accounts are good too, but they go through a broker first, which can sometimes lead to slightly wider spreads or a bit of slippage. So, if you want the best trading conditions, ECN accounts are usually the way to go. That's a personal opinion though. You need to make your own research and choose something that is compatible with your trading style
It depends on the size of your lots. Most ECN brokers operate in a mixed manner, meaning they themselves are direct participants of the same ECN network, i.e they are also market makers at the same time. That means, that the tightest spreads are offered are filled by themselves. If you have a look at the level II market ladder, you will see that at most of the ECN brokers the tightest speads have an offered volume of ca. 1 Million units of the currency. This is offered by the broker itself. This means, that if you trade lots of one million or more, probably you will get partial fills or you will have to pay wider spreads. If you trade 100K standard lots or less though, you will have the tightest spread offered, but probably be filled by the broker himself. Orders over 1 Million will be forwarded to the real ECN market.