Are you confused about the functional differences between TMG (Threat Management Gateway) and UAG (Unified Access Gateway)? You’re not alone.
Thomas Schinder (MSFT) sums it up nicely by mentioning that UAG is designed to be an Enterprise solution for remote access into internal corporate resources without supplying any outbound access.
TMG is designed for both inbound and outbound access but at a more comfortable price point for SMB’s.
Here is a good TechNet thread where numerous Microsoft’ees chime in with their opinion:
Mike Howells's Blog 