Short answer: I don't know
Long answer: It depends. Both the analysis software have their own place. They fit a specific need. It's like saying which is better between solid propellant and liquid propellant?
Both are useful for a rocket. Solid propellant gives the initial thrust, while liquid propellant propels it out in space.
Likewise nastran and ansys have their own need.
But if you ask my choice, I will prefer ansys. In my opinion, the Ansys has better range. In an aerospace domain, many tasks are impossible without it.
No hard feelings for nastran though, but ansys is what I like and recommend.
That being said, as I always point out, softwares are just tools, a means; real thing is you.
So focus on increase you knowledge and then any software will be useful!
Find More on ansys on this blog by this link http://my-aesi.blogspot.com/search?q=ansys