full duplex and hearing what you are recording are two different issues
full duplex, means that you can hear the drums while you record the guitar, but you won't necessarily hear the guitar too, although it is being recorded. the million dollar questiong is: while you hear the drums, are you still recording the quitar? if you want to hear the drums and the guitar while recording the guitar, then you want full duplex with a patch from the IN to the OUT on the recording track.
my soundcard is configured to use an ADAT lightpipe (8 track) as well as stereo SPDI/F connections. This way, I can monitor using the SPDI/F, while recording on the lightpipe. The soundcard (Frontier Design WaveCenter) has an internal patch bay that allows you to listen to the track as it's being recorded.