For sure, use 24/48 to record in. Newbs don't usually realize how important it is to get those extra bits. It's not just about how the recording sounds, it's how much information is there to let you tweek the recording afterwards. Minor miracles can sometimes happen in post with digital recordings (EQ, normalization, etc) if the data is there. If it's not there, nothing you can do.
In that regard, an audio recording is no different than a digital photo. It doesn't look any different when viewed from a distance, but when you zoom in, it's night and day. So, up to a point the more data there is the more you can do with a recording. Most of us do agree though that beyond 24/48 for audience recordings, it's mostly overkill.
The reason it's better to leave AGC off is that when AGC kicks in it can affect the sound of a recording...you can sometimes hear it kicking in and when you do, it's not always pretty (although level controllers on some recorders, like the M10, are fairly transparent). Since it's easy to adjust levels upward in post, the better option is recording at 24/48 with your levels set low so that you never have to worry about your levels peaking over zero. As a compromise, some people will run low levels AND set AGC on just as a safety valve in case there's ever a ridiculously loud passage during the show. that's extremely rare though.