I would suggest getting general admission for Saturday, and possibly a grandstand for Sunday. If you get a grandstand seat for Saturday, I would suggest that you may feel like you're missing out on some of the action you will see by walking the track. Walking from turn 1 to turn 7, and watching turn 17, 18 and 19 is a great experience. The corner speed is crazy, especially from the smaller bikes.
For the race, the best grandstand is between turn 15 and turn 16. From there, you can see the breaking zone after the long back straight, and watch the bikes come through turns 12 to 16. There is also a jumbotron in this area so you can follow the race.