My signature tells you who I prefer, when it comes to airlines. But being from Minneapolis (and somewhat in the industry) I can tell you all about NWA.
First of all, their mechanics are on strike. They have been on the picket line since August of 2005 when NWA threatened to outsource over 50 percent of their jobs. Pilots are currently voting whether or not to approve a tentative agreement with the company to outsource small jet flying and wage concessions larger than Delta's pilots. The NWA pilots union has overwhelmingly approved a strike if the company voids their current contract.
As for in flight service, you get what you are paying for. Your online ticket gets you a seat in the coach section of the airplane. NWA now charges $15 for preferred coach seats such as aisles and exit rows, so if elbow room is an issue, be prepared to pay. There is no video or audio entertainment on NWA aircraft. Unlike carriers offering tv, movies, or audio only programming, you are left to look out the window or read a book. If food interests you, you can spend $1 on a 3oz bag of trail mix. Longer flights may offer larger box meals for $5, but you will not get any free pretzels, peanuts, or cookies on any flight. Of course, beverage service is still complimentary, but NWA is known to make some shorter flights limited to 6oz of frozen OJ or a glass of bottled water.
With NWA, you get what you are paying for...there is a reason they are one of the cheapest ways to fly right now. I would strongly suggest taking American over NWA any day of the week. A quick look shows low fares out of Reagan on US Airways and Midwest, both of which are decent companies. If you have any questions, feel free to ask or PM.
Edit: Re: 50 seaters
On nearly any major carrier you will come across these regional jets. These are pure jet powered planes just like your A320 or B737, only much smaller. You are sacrificing cabin comfort and overhead bin space when flying these planes, but not safety. These planes are held to the same maintenance standards as the mainline aircraft, and typically have better safety records over mainline (all of which are stellar).