Good question. I often wonder the same thing aside from I like the looks on that bird.
The Prowler has attack capabilities. More so than the S-3. You have your crew right there with you as well. The E-2 is a pretty sweet deal too. My friend took me in one and a few rides in the sim. Pretty cool!
I guess it's all a matter of which community, or lifestyle, you want to be a part of. Obviously, if you get a P-3, you won't be out on sea duty. Which is good if you've got a family. Lots of things to consider, but, it all comes down to the "needs of the Navy" anyway. If I join, I want it to be because I want to serve my country first. Then, I want to get paid to fly and wear green pajamas to work everyday!