Titanium is a lot stronger than steel, but it also carries a hefty HEFTY price tag. Also, titanium cable cuts almost as easily as steel cable. If you go titanium, I'd suggest a titanium chain...and those start off at a several hundred dollars...once you move to semi-precious metals you're not paying for a length of metal, your paying by the gram.

Large diameter chro-moly chain will break most common bike thief's harbor freight bolt cutters before they even scratch the finish. Remember, unless they're professionals, they not likely to carry big time gear.

Edit: If you're not familiar with chro-moly, it's one of the strongest types of steel out there. NASCAR, NHRA drag, Monster Jam, and International Rally Racing all use roll cages made of chro-moly. It's strong and it's fairly cheap. It's also used to produce most car frames these days.