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.