One point - all military training aeroplanes are low wing. Thus it's relatively easy to look into the direction of the turn irrespective of bank angle.
But many civilian trainers are pieces of cr@p like the C150/152 with a high wing and a pretty dismal rate of climb. Fly one of those at 15 deg AoB through a continuous 180 deg at the upwind end of the circuit and you'll be blind in the direction of the turn for quite a while as you can't see through the wing! Certainly you'll be blind for much longer than you would in 2 x 90 deg turns at 30 deg AoB in a rectangular circuit.
Personally I'd prefer racetrack circuits with a downwind spacing such that the runway is always within gliding range. Realistically that's unlikely though. But those ludicrous Bomber Command circuits flown by many (e.g. as at White Waltham) have no place in light GA flying.