With the Sideshow set, you get Jabba looking EXACTLY like he did in ROTJ, a really great display piece, and an awesome collectible. It's only downside is, there are fairly less characters to add to him.
With Gentle Giant, you get a decent looking Jabba, but it looks fairly off from ROTJ. On the plus side, it's smaller, there's a Max Rebo band counterpiece, also a newly made Slave Leia made to go with it as well and a Bib Fortuna.
GG offers a more encompassing scene, Sideshow offers you a more accurrate appearing scene.
If you have the space, since they're roughly the same money, I'd say you're best getting the Sideshow set.