I get asked this quite a quite a bit and I couldn’t find a consistent answer. My colleague, Steve Scholz, provided me this useful feature matrix as when to use UAG for publishing Exchange 2010 and when to use TMG:
To summarize, either UAG or TMG will provide reverse proxy publishing for Exchange 2010. The only differences are specific features are lit up with TMG and UAG. For example, if you need client cert based auth go with TMG but if you are worried about OWA leaving residue on public machines you will have to go with UAG.
Both UAG and TMG leverage the same protocols, same certificates, and similar publishing approaches.
Here is a whitepaper explaining UAG vs. TMG options with further details: