One piece toilets tend to be slightly quieter, but are heavier (which is only a factor in carrying it up to the bathroom and installing it). For toilets with the same technology, they should all perform about the same, so it comes down to style and price points...you'd probably be happy with any one. The e-max ones have been reported to perform as well as the 1.6g versions. They're calibrated better so they don't overfill the bowl...they use about the same amount to actually dump in and flush, on the 1.6g versions, they run a little longer and overfill the bowl - it doesn't really help the flush.
Did you read Terry's reviews off the home page?