I've played FF IV on four different platforms, and I rather enjoyed the DS version.  I'd go with that, honestly, if price is a factor.  Honestly, given the choice between the GBA cart and the DS version, I'd probably pick up the DS version again.  
The problem with the original FF II version is that it's a little dumbed down.  Some of the character abilities were removed and the difficulty was a little toned down when it was brought over to the US.  These differences were also corrected in the GBA version, of course.