While round would seem to be the best shape, in practice I can take a drilled crappy coil crammed into a stopper and shove into the backside of a valve on a rectangular cooler and get 85%+ efficiency each and every time. I believe the problems with the round are size limitations, costs, and most people end up with some channeling whether they realize it or not.
Go with a rectangular, if the batch gravity and size is small, you can always use a bottling bucket with a coil to achieve an adequate grainbed depth if fly sparging.