The Fireface for sure. Though my actual vote would go to the Multiface II for expandability and of course insanely low latencies (my Multiface I does 0.7ms due to the HDSPe card). The great thing is that the Multiface comes with no pres, so you can pair up whichever preamp you want with its decent converters, or if you decide to expand in the future, just hook up more converters via the ADAT lightpipe.
But whichever option you go with RME, you're pretty much assured great drivers and support, unlike Digi.