Darkcoin seems to have staying power and volume, as does Bitfinex. Seems a logical choice to me.
What I would like even more than "darkcoin at bitfinex" is the ability to trade any pair on any of the exchanges that we're already connected to.
Even if you have no backtest data and are not recording data. Just live access.
I'd love to write script that traded through NMC PPC LTC BTC and USD at BTCe