The fixed spread broker may Requote your trade, slide your entry/exit a couple pips, disconnect/freeze the platform - market makers have all kinds of tricks that they use to give them the edge over you since once you shed THEY win. ECN's only make money from commission that they don't care if you lose or win. (in fact they hope you win so you trade more and larger)
Those tricks could cost you tens of thousands of pips over the course of a trading year. For many those little pip slippages on entry and exit can be the diff between profit and loss. Finding a honest...