Definatly backstage. As a
TD/SM myself I've never managed a show watching it (quite honestly I can't remember the last time I worked on a show and saw it from the front). At the same time however I always delegate a lot of small tasks to my ASM for
practical reasons (I graduate in 9 weeks). We're always connected with radios/walkie talkies to the booth and the followspot operator for cues which I prefer as I would rather be able to cope backstage with the actors and my backstage running crew than hear about backstage problems over a radio.
However until tech rehearsals start I usually sit in on rehearsals to get to know the actors, take notes on behaviour that might be a problem, talk to the director,
etc. That way I also essentially get to see the show =)