Neo4j is an open-source graph database (GDB) well suited to connected data
Titan is a highly scalable graph database optimized for storing and querying massive-scale graphs containing billions of vertices and edges distributed across a multi-machine cluster
"When the data is on disk titan is faster than neo4j cause it has a better disk representation"
from question "How does Titan achieve constant time lookup using HBase / Cassandra?"
"Because it seems you re going to deploy a cluster i think titan is the better choice unless you re willing to pay for the enterprise edition of neo4j to support clustering"
from question "Using Rexster and Titan Graph DB for scalable applications"