LMAX is a good choice overall. Though, sadly, they stopped servicing Canadians so I can't trade there myself anymore.
My only concern with them would be that they aren't yet profitable as a business, and most of their original investors have long since bailed (GS ran for the hills long ago, and they generally know a good investment when they see one.)
They even had to do a leveraged management buyout so their last major investor (betfair) could reduce ownership without some vulture gaining control and breaking things up. Actually, to be fair I have no idea why they did the MBO, but it was leveraged, and betfair did want to reduce their ownership from a majority/controlling position to something more passive...
Don't get me wrong, I'm not saying this to put down LMAX; I rather like their model and personally want it to catch on in the spot industry.
I bring the profitability and investors bailing thing up since they are acting as both the broker (LMAX Exchange) and the MTF/ECN (LMAX MTF.) So as a whole, they need to be in the black or else they'll need to change something drastically, either on the execution side of things or on the broker/client side of things. And I wouldn't want to invest my time and capital into an environment that will either not be around or drastically different in a short time.
Of course, this matters more if your edge can be found in part within the exchange membership itself (like short term scalping, cross market arb, anything that depends heavily on the execution conditions staying static..) if you're position/swing trading then it doesn't matter so much... but then again if you were swing trading then asking about electronic venues with all the OP's requirements doesn't matter in the first place, so I'm just speaking in context of the thread.
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IB's strength is in the multi-asset class accounts. That's always been my main interest in them.
They have a decent forex offering but the biggest point of pain is the minimum order size and minimum commission charge making exact risk management kinda hard. Forget trading less than a half lot unless you don't mind your total cost going up drastically. This matters to traders who want to define exact risk down to the dollar/penny on some trades, or long term traders who'll use lower leverage but be holding for longer swings... or heck, forget about basket trading 10+ pairs at once for a net exposure.
They also rape you on the cost of carry... I remember when the EUR/CHF peg was being defended and a bunch of retailers went long, most brokers I worked with were paying interest on longs even with the thick markups they pad to swap rates, but IB was charging both ways (long and short positions,) and it seemed excessive.
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I also like Dukascopy, who I have first hand experience with using. They've been doing a lot of work lately tightening things up on their ECN. Americans can even access them through a white-label in the US (Alpari and FXDD... both companies I would never recommend for their main services but when you use JForex through them it's all Dukascopy, they just do the intro and take your deposit.)
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And for liquidity taking only (non-participating in the exchange itself, but executing on a raw quote feed from 18+ liquidity sources) there's Pepperstone -- MT4 based, most will bulk at such a suggestion compared to HotSpot or IB, but the quotes you're getting are right from the interbank, liquidity is as good or better than places like LMAX, and it takes a lot less capital to get started (plus, they don't make it painful to trade smaller lot sizes, and treat all order sizes the same.)
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Other than that, I've been looking around for other alternatives..
Never heard of solid-fx OP brought up.. not really sure what drew you to them either, they don't appear to be offering anything that stands out from the herd.
