Last year I was walking the upper deck with Larry Lucchino, Red Sox exec, who was instumental in building Camden Yards, the Padres fine park and redesigning Fenway. Aside from architects, I don't know who would be a better expert on ballpark design. And, once he left the Orioles, where we constantly had enjoyable arguments __he was EBW's designated bulldog__ we've had a lot of good talks. Anyway, he thought, and I agree, that the most distinguishing feature of Nats Park, which is otherwise a very nice, but not quite Top 10 stadium, is a truly wonderful upper deck.
You can't go wrong up there. Kasten deliberately wanted to have good seats and great prices for more than 10,000 people. That way you grow you fans base; you don't just see how many $60 seats and suites you can sell. The 300 level is excellent, top price ~$24. Maybe 2,000 seats have Capital views. But thousands of the $17 seats have views as good as the (very high) press box. And I've been known to go sit up in the $11 seats when I'm not on deadline just because it's breezy on hot nights, very panoramic.
BUT you have to like the upper deck! Be serious, it's no box seat. My wife doesn't like heights, so it's out for us. But I love walking around the concourses and all the views.
The Nats would much prefer that you buy the more expensive seats lower down! The 200's are excellent, especially between 3rd and 1st. But ~$50.
Until they turn those garages into something easier on the eye, I would touch the (higher priced) lower deck if there was an alternative at the 200 level.
But, for value and sweep of view, in all of baseball, it's hard to beat the 300's, 400's and even some '500s at Nats Park. Washington just isn't as beautiful as, say, San Francisco. But there's no site in this area that makes better use of what D.C. and vacinity have to offer than the upper deck at Nats Park. (You'll note I made no mention of the lower bowl. In an era with so many magnificent parks, it's just okay, though I like the RF bleachers.)