As far as I understand these two types of order execution are quite similar. They both represent the direct execution, meaning that the trader gets the access to the liquidity providers in both cases.

However in the case of STP execution, the trader gets access to the liquidity providers of the particular broker. I suppose FXCC has some kind of agreement with the number of liquidity providers, and they provide liquidity to Fxcc customers.

Whie ECN execution implies that traders' deals are in the liquidity pool for everybody to access. Not only big liquidity providers and market makers, but also retail traders like you.

Hence why Fxcc chose the STP execution, as it is better in terms of offering the best prices of the assets.