For the National Guard it depends on the type of units in your state. I am in the Alabama Army National Guard and was Infantry in Regular Army prior to joining the Guard. My state has only one Infantry BN in the state so promotions are slow. LOL we have more Special Forces slots in my state than Infantry. Our Armor and Artillery units have been cut to almost nothing as well. I would suggest taking a look at the type units in your state and go with what they have the most of. Another suggestion is to go with an MOS that are found in almost every unit like supply or NBC.
One problem with going with a certain career field in the Guard is they change a lot. The Guard unit in the small town near me was an Artillery unit when I moved here 12 years ago. It soon changed to an Armor unit then changed to a Chemical unit and now is being converted to an MP unit. The Guard is constantly changing to meet the current needs of the Army. So if you stay in the Guard for a career you may change MOS's several times before you retire.
When I first joined the Guard what I did was more important than quick promotions. As I have gotten older money is more important and so faster promotions is more important.