If it comes down to just those two choices, I would have to say TradeStation simply for the data quality issues. All of the MetaTrader brokers are notoriously bad when it comes to data quality - inaccurate close times, Sunday candles, non-conformity to interbank trading hours, etc. That's just the tip of the iceberg, you also have to factor in all of the spread games inherent in the dealing desk environment. Case and point, run the same system on any two different MT4 brokers and you'll come up with completely different results (sometimes drastically different).
TradeStation also widens their spreads (they use Gain, but add 0.5 pip to Gain's spreads). Since they charge the same commission, essentially this is outright fraud on behalf of TradeStation, but in the wild west world of unregulated FX, anything goes.
What I would really recommend is using eSignal's feed (aggregate of 140 banks) and import that into any other advanced system software (ex. Amibroker) and then trade with an ECN that supports API trading (IB, Dukascopy, etc).