What is the concensus on when to use one of these tools adversed to the other? I find Subsonic very useful in terms of getting things done quickly, but on large projects it
I think you pretty much nailed it. Subsonic generates code, so your business objects will be reflective of your database structure. nHibernate uses mapping files which map your business objects to the database so your objects can be structured however you like.
How large of a project is this? Will there be long term support needed? Is the cost effectiveness of Subsonic going to offset any potential scaling issues?