Yes I am not surprised by what Sighandnod is saying, because I realy dig the SU700 as well. A good realtime/live machine with sampling is the Electribe "S", if you expand it to 64MB.
KORG Electribe S
Yamaha SU700
NOW dont get me wrong.. this is good for realtime stuff, but if you are a studio guy, go for a Yamaha 4000 or for an EMU (any you can afford), they are wonderfully stable, but not as good for "stage" performance
Ahh by the way... with those machines above, you do NOT need a computer, because their built-in sequencer is quite good, yuou can even trigger notes on external MIDI devices. Just so you know
good luck
by the way.. I think you are believing that a DJ who has synthesizers and drumboxes on stage are not composing.. but there is NO difference, they ARE composing music, and preparing their patterns BEFORE they go on stage.. It is live music, not mixing as you imagine it. Just wanted to make something clear.
If you get any of these models, expand them to 64MB, it is very important. They come with 4MB or something ridiculous.