a duo is for recording purposes. it has two mic preamps, a couple outs, a couple ins...it can also act as a standalone A/D convertor. add to that ASIO, MME drivers for low latency, and its a great recording unit.
im not familiar with the extigy, but as i understand it, it is geared more towards gaming and output - not necessarily input. personally, i would never attempt recording anything with a soundblaster. maybe thats bias...but in terms of outputting sound, an extigy works well.