The geometric network is ideal for utilities and network dataset/network analyst for transportation networks.
The main difference is that geometric networks have the concept of "flow". i.e. something moving along the network in a defined way. So you can perform tracing upstream/downstream. It does not have concepts such as turn restrictions, turn delays, street name for directions, etc. Network dataset supports these navigation type capabilities. Network Analyst is a licensed extension that operates on these network datasets. Geometric networks are part of core ArcGIS.
Hope that helps to clear up some confusion.
Regards,
Jay Sandhu