I like base4 more for subjective reasons than rational ones. I think in the end they are pretty much the same. Base4 does abstract the data on a more conceptual level. Furthermore base4 has built-in support for files which I think is pretty unique for an OR/mapper.
It runs on remoting so you can run multiple base4 datasources installed that run and you wouldn’t know in your application because you’re just working with classes.

But to get deeper into what base4 is and the differences between base4 and NHibernate i will have to refer you to Alex’s blog.

Another reason why I like base4 more than NHibernate would be because the guy that wrote it lives in the same country as I do and is a friend. Whereas I don’t really know Ayende except from his blog posts and that he’s an incredibly smart guy who doesn’t seem to sleep. I mean how many hours are there and he’s maintaining rhino mocks, rhino.commons, brail, nhibernate,… and does a normal job too.

I hope this answers your question somewhat. I do think that if you keep up with my blog I’ll come up with a way of using base4 that is faster than the way i’m doing it now and very similar to the functionality of active record . I just have to run a couple more tests on a class when I have some time.