Opinions Sought: National Guard or Reserves?

The best advice is to talk to HR at the state headquarters of the states you are interested in joining. Transfers with the Guard system is generally pain free, but I went to the Reserves too and didn't have any issue there either. Do need to be aware that you will need to get copies of all you Guard stuff since it is my experience and apparently still true that NGB system and Army Reserve system don't share information. Cpt. Hignight is correct about promotions in the Guard. Depending on the state, but the Officer ranks tend to bottleneck at Major, depending on your Branch.
And just as a note, Southeastern PA would also be fairly accessible from that DC area.

Specifically for a 35 series, I am no help. However, in many Guard units, the further you move up in rank the more competitive promotions become because you are only competing with those in your state. (I have known a couple of 10 year LTs because of such.) You have sometimes have to wait for somebody to die or retire before you can get the spot and promotion.

The Reserves you compete for promotions across the entire enterprise and can promote into units that have vacancies across the country so you don't typically get bogged down. The Reserves will also pay, as long as there is money available in the budget, for you to travel to and from drill as well as LIK (hotel/barracks) while in attendance.

Just something to think about.

If reserve soldier moves to another unit, it is considered a PCS move?