partly depends on what you're going to see. If it's a big production, and if the cast are miked up, the circle should be no problem. If it's just two people speaking in their own voices on an empty stage the upper circle may feel a bit remote. You might also want to check out sightlines in the back of the stalls - sometimes pillars partly obscure the view from some seats.
I always choose the stalls now, since I had a terrifying experience in the upper circle at the Palladium - I did'nt realise how high it was - I was so terrified I could get out of my seat. In fact I am sure thats what started my fear of heights - I was fine before that !!!!!!!!!