You can choose as per your current situation and future plans:
1. Reserve:
- If you have a good job or if you think you can land a good job after getting citizenship
- Currently in school and planning to graduate in future
- if you are planning to study further and Army want's to pay little portion of your education

2. Active
- Working Army as a full time and getting out from there after 4 years ( 6 years comparing to Reserve)
- You dont care wherever you are station
- Want to experience Army life
- Education benefit is really good but after getting out of Army (Paid for everything including housing and books)

Experience guys can add input