Well, given that you know you've asked a vague question and you're probably expecting vague answers, my response is: it depends (as you well know :)

What will cost less money? Would rewriting the app or writing a wrapper be less expensive? Also, if you rewrite instead of wrap, what is the potential impact if your code does not work properly? I wouldn't be too happy to hear about programmers rewriting perfectly good ATM software, for example.

Of course, if this is just for fun and you have spare time, rewrite away! Ain't nothin' wrong with havin' fun with what you do.