[hughissuper]

As far as boot camp, AF is about 6 weeks. Army is 9 weeks.
As for the ASVAB, the test they have to take, that's required for every branch of service. It determines which branch of service and the job. The ASVAB is basic algebra and English. If your nephews did decently in highschool there's no reason why they shouldn't do well on the test. There are practice books available.
As far as benefits, each branch of service offers basically the same thing. The GI Bill is available to everyone in the military, the amount available is determined by the branch of service. Health insurance and pay is pretty much the same starting out.
What they might want to consider are the more mundane benefits. Air Force bases are much nicer to live on than Army bases.
More specific information is available on the internet. You should google for the specific answers to all your questions and check several different sources. People on a forum are going to give their own opinions, which may not be reliable or informed. You're not going to get the full picture on here. Google what you want to know.