The advantage of the ESL is that the ECU retains all of it's safety features i.e limp mode when it detects a fault and closed loop knock control.
It fits into the ECU via dedicated connection points that Subaru designed in to get around rally regulation. These stated the standard ECU must be used, but didn't state you couldn't put extra stuff inside
Only the very expensive aftermarket ECU have such features and they may even be bolt on extra's (I'm not up to speed with the latest aftermarket). You probably won't see change from £1400 if you want similar ECU technology to the OE ECU.
Apexi are great. They were great in the day and still are today. I've been running mine since 2006.
They actually run smoother than the OE ECU during cruise and you have the benefit of the hand controller to keep an eye on things.
Tools/displays to keep an eye on things with aftermarket ECU are again going to draw a few more £££'s from your pocket.