Fortunately, you cannot really go wrong with either choice - Air Force or Navy. However, there are significant differences between the two, as Bob Norris has so eloquently pointed out in an earlier post.
Personally, I think it quite notable that most of our recent Presidents served in the Navy: Kennedy, Johnson, Nixon, Ford, Carter, and Bush Sr. I also suspect Congress is populated with more former Navy and Marine service members than any other.
Since you are interested in the astronaut program, consider this: Even with a smaller pool of aviators for selection, the Navy still has the highest representation of current astronauts.
"Of the current 108 active astronauts, there are 23 members from
the Air Force, 1 from the Air Force Reserves, 6 Army officers, 1 Coast Guardsman, 6Marines, 27 from the Navy, and 3 from the Navy Reserves."
http://www.nasaexplores.com
There are very real and important reasons why our nation's leadership, the astronaut program, and many other august groups enjoy such high, U.S. Navy representation. May you be fortunate to explore those reasons, personally.
Many years ago, I confronted a similar choice as you. I wisely chose Navy. While perhaps a slightly more difficult path, it has proven to be far more exciting and rewarding, both personally and professionally, that I ever could have dreamed.
With absolutely no regrets, I would again jump at the chance to do it all over again, in the Navy – in a heartbeat!
"I can imagine a no more rewarding career. And any man who may be asked in this century what he did to make his life worthwhile, I think can respond with a good deal of pride and satisfaction: 'I served in the United States Navy.'" (John F. Kennedy's remarks at the United States Naval Academy, August 1, 1963)
Good luck in your quest!