I stood at an Amy Winehouse performance and it seemed the right thing to do especially because the venue had a bar but I sat at a Leonard Cohen show which also seemed appropriate because of the slow nature of many of his songs. Depends on what's playing.
Standing, I like being able to move around and dance if I feel like it I like being at the very front if it's somebody I like. I find sitting in get to bored and I can't focus properly if it's at a bar or a small venue and I feel like I want to drink then I prefer sitting and just relaxing.
If I'm seeing something 'upbeat', then definitely standing. I've seen Avril whilst both seated and standing and I had just as much fun at both. If it's something theatrical, I'd probably prefer to be seated so I can see everything better (but then I just buy tickets for two gigs on the tour so I can sit for one and stand for the other because I have 0 self control with concert tickets). I don't think it makes too much of a difference to me, as it's been said, you can always stand even if you have seats
'I tried to hide but I still believe, we, that we were always meant to be, but I could never let you go, no Hush, hush now'
'You can't plough a field by turning the soil over in your mind'