There are plenty of answers regarding QueryOver versus Query here on Stackoverflow but in a nutshell:-
QueryOver is a strongly-typed version of Criteria, and is more
NHibernate specific. Pretty much anything you can do in ICriteria can
be done with QueryOver. In the golden days of ICriteria NH2 you always had
to cast, hence this is why now you need to cast at the end of the
chain back to an int.LINQ (Query) is a standard query method that works on IQueryable that
doesn’t need explicit references to NHibernate and can be considered
more ORM agnostic and therefore follows the linq standard. As you
rightly pointed out you do not need to cast to an int as you are
selecting into the result the customNumber.
I would be very surprised for your simple example if the generated SQL was very different.
I was a big fan of QueryOver but as the Linq provider is getting more mature then 95% of my queries I use Query but for some Nhibernate specific stuff I resort back down to QueryOver. Either way I recommend using a profiling tool to see what you can live with.
Refs: Tradeoffs or versus and versus