+1 for Ableton Live
Nothing is more flexible for triggering samples live.
When playing live, we use Live with a pair of small MIDI controllers (Keith McMillan 12-Step and QuNexus).
This allows us to stay a 4-piece, work some keyboards/samples into the show, and not have a large keyboard consuming precious stage real-estate.
You can setup advanced click-tracks, play virtual-instruments (seamlessly switching between advanced splits/layers), trigger samples, instantly switch songs (with tempo changes), and much more.
If you haven't already done so... watch some video of what folks are doing with Live.