I think Rumpel’s decision to go with this particular method derives from two basic things. One, of course he doesn’t want to deal with the consequences of the curse. That’s a given. But most importantly, in order to cast the curse, he needed to rip out the heart of the person he loved most. I highly doubt he would have sacrificed Belle for the curse, and he couldn’t use Baelfire’s heart because, uh duh, the whole curse was about finding and reuniting with Baelfire!