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
Consider your team and project size when considering ActiveRecord.
In my experience, ActiveRecord is an abstraction on top of NHibernate that starts leaking like a sieve when trying more complicated scenarios.
If you have a moderately to heavily complicated or non-straightforward schema, stick with NHibernate. You can slice and dice it to near perfection.
The other place you might get into trouble is when you need a moderately complicated query. ActiveRecord hides a lot of NHibernate's implementation... but you'll need it for a complicated query, which will become very difficult if you're completely unfamiliar with HQL. Be careful team members don't just hack away at the edges instead of learning NHibernate and HQL.