Since Star Wars Rebels is more serialized than Clone Wars, dropping into “Twilight of the Apprentice” (which is comprised of the final two episodes of Season 2) will be a little more complicated than dropping into the Siege of Mandalore arc.
If you’re still undecided about Rebels or are just a Star Wars fan, watch “Twilight of the Apprentice” anyway as a taster.
If you plan on watching Rebels, I recommend starting at the beginning and working your way up to “Twilight of the Apprentice.”
(“Twin Suns” in Season 3 is worth checking out as well, if you’re not afraid of spoilers.)