If you have moved up from the pure Retail level in FX, but have not yet arrived at the pure Institutional level in FX, then it does not get much better than BARX FX.
I consider BARX FX, one of the preeminent "Commercial" grade trading platforms and liquidity sources out there. You'll need about $300k to open your account. Their representatives speak in fast, straight forward terms - so when you call them, call prepared, having done your homework about what it means to trade at this level. This is not Autobahn FX, but that's Institutional anyway and you'll need millions to set up an account.
Best bet for high level Institutional: Find yourself a good Prime Broker who runs FXall. You will avoid much of the legal headaches associated with getting on platforms like Autobahn and you will have a trading platform that is designed with the Trader in mind - which means, flexible order types and a full spread API for AutoBot executions. Your liquidity through FXall will be some of (if not THE) best public facing FX liquidity available to the individual/private Comercial/Institutional Trader.
The term "ECN" has been misused more in this industry than in any other trading niche I know. STP is a "function" (or, can be) of an ECN. An FX ECN is in simple terms, an order routing, parsing, integration, aggregation and matching, OTC transaction distribution hub. STP, assumes a middle layer between your desktop client and the "real interbank market" that is NOT infiltrated with human intervention -aka- a Dealing Desk. However, technology being what it is today, the so-called "Dealing Desk" does NOT have to be manned by humans. It could very well be a Virtual Dealing Desk, driven by mathematical algorithm.
Technology will enable Retail FX to look on the surface more like Commercial and/or Institutional FX. Thus, the ECN and STP will merge into a EVSTP model, if this segment (Retail) continues to grow. No doubt, your "Broker" will pound themselves on the chest and proclaim that they now offer "Institutional Grade STP," or something equally as ridiculous and bloated with marketing hype.
Today, this should not be your focus - because it is out of your control.
Today, as a Retail Trader, your focus should be whether or not you trade with a real Bank, or not. That's probably the single most important question you can ask yourself before opening an account. If the answer is no - then your next Retail question should be, am I trading somewhere that offers me at the very least, Currenex type liquidity routing. If the answer is still no - then, you need to seriously question whether or not you are trading Retail FX through the proper channels, because the only thing left will be Buck Shops like FXCM, OANDA and the list goes on, seemingly forever.
That should just about tidy things up here.
