Is it more likely to vomit or poop blood from an ulcer? which is more concerning? and why would your body do one or the other?

Vomit: Varies with volume of bleeding. Small amounts may just pass and turn stool dark or black- tarry. Large amounts may irritate enough to cause vomiting bright red blood. Either is bad and needs attention!

Depends where ulcer: Brisk bleeding may result in hematochezia (passing blood per rectum), but more likely present as hematemesis (vomiting blood) or melena (black stool). Patients with bleeding ulcers above the end of stomach (pylorus) will likely vomit blood sooner than it will be seen as rectal effluent, whereas ulcers bleeding far down from the pylorus may first manifest their bleeding in stool. Both are serious.