It really comes down to a couple of things -- how (a) close you are to (b) one that does what you want to do. "Close" because you have to make the drills. While some air guard pilots may be in a unit several hundred miles away, they are usually commercial airline pilots who can ride a jump-seat to their meetings.
For Army it's different, and if you are just enlisting, then you need to think about how far a drive it is to a unit that is a Branch of whatever specialty you want to be in. Otherwise, you almost have to do what your closest Reserve unit does -- if it's Infantry, or Engineers, or MP or Medical Service Corps, then that's what YOU will train in, and what you'll do. In some states, smaller Reserve units are parts of larger Reserve units in the same state. So, your local reserve unit may be an Artillery company that is part of a larger Artillery battalion that may have units across the state, or even several states. Or maybe your local unit is an Engineer unit attached to a larger Infantry battalion. Make sense?
Realize, too, if you're doing this to try to increase your chances for West Point, I'm not sure you'll necessarily be better off. You may increase your chances for USMAPS but you will also increase your chances for a deployment -- and while you can be in the Army Reserve and still go to college, if the unit deploys, you WILL deploy with it. If you're thinking ROTC, then there is a Simultaneous Member Program, but again, all bets are off if your unit is deployed.
I suggest you get some solid counsel before you commit. Check with your Academy liaison officers on your thoughts too -- Best of luck.