What is worse thoracic or abdominal aortic aneurysm?

Size & location. This really depends on size of the aneurysm, the bigger one especially above 5-5.5 cm will have higher risk of rupture and death. Also location so if aneurysm involves major branches to brain or arteries to kidneys or intestinal arteries all this will make the surgical repair more risky.
Why choose? Prior to endovascular stent graft treatments of aneurysms, open thoracic aneurysm repair was definitely more risky than repair of the more common abdominal aneurysm. Now, however, when the anatomy is suitable, treatment of either of these entities can be accomplished with small groin incisions and an overnight stay in the hospital.