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 believe you should stick to one which you can utilize the best. The ultimate goal is the productivity and good performing quality code. If you know SubSonic in and out then stick to it and if you know NHibernate in depth stick to NHibernate. This is very subjective question. You should also consider the fact that what your team-member are having expertise with. If you are good at it you will be able to easily maintain it.
I've seen large projects using SubSonic whereas NHibernate is already famous and used widely.
The decision of picking ORM does not solely depend on the ORM itself.