When I was in the National Guard it was as a translator - not a dentist. So I can tell you based upon conversations with others and what I saw as a non-dentist.
As a dentist in the National Guard you will do more administrative things and training on your "one weekend a month" drills. Your chance to be a dentist will mainly come during your "2 weeks/yr". These 2 week ATs may be humanitarian missions in other countries, or they may be at regular Army bases during the summers.
Life as a National Guard Soldier will allow you to maintain your private practice, as well as provide important perks like health care that you can get for you and your family much cheaper than otherwise. If you stay in 20 yrs you can also get a retirement when you turn 60 or 62.
In the National Guard if you deploy it will be for 90 days + the time to get ready and time to turn everything in when you are done. This may be up to an additional 1 month.
The pay can be found at
www.dod.mil/dfas - click on military pay tables and scroll down the page until you see the part for "drill pay" the amount for 4 drills is what you would get paid for your weekend - currently $472/weekend for a new Captain.
Joining the National Guard, they can either pay you a stipend during school (not tuition), or you can join after you graduate and get a sign on bonus or loan repayment.