By the AIP statistics, yes, astronomy programs are almost always less competitive (measured by admissions rates and average scores) than physics programs, including at schools where both departments are top 10.
The one major potential exception to this is in fact Princeton, whose astrophysics graduate program is very small, very good, and very selective.
Purely anecdotally, I've found astronomy types on average a little less capable than the general physics population. (I'm a physics student who does research in astronomy, so I have at least some perspective on this.) Of course, there are some brilliant astronomy students.
If you're applying to a physics department, I doubt your subfield will make much difference one way or the other, unless it's a program that has few or no professors in your area of interest (and you wouldn't be foolish enough to apply there anyway). And heck, at my school, which has a large and excellent condensed matter program, the astrophysicists seem to be better funded these last few years; recent NSF and DOE budgets have not been good for everyone.