Short answer: they're not really interchangeable — they target different problems

Alginate

• Mechanism: forms a viscous "raft" that floats on top of gastric contents, physically reducing acid reflux into the esophagus. Combined with an antacid, it can also neutralize/displace the "acid pocket" - Yamada's Textbook of Gastroenterology.
• Primary use: GERD and reflux symptoms (heartburn, laryngopharyngeal reflux), not primary ulcer therapy. It has no antimicrobial activity against H. pylori and is not part of any ulcer-healing or eradication regimen.
• Good for symptomatic relief with a strong safety profile, but does not treat the underlying cause of most peptic ulcers.

Bismuth

• Mechanism: bismuth salts coat the ulcer crater by binding to mucus/damaged protein, may increase mucosal prostaglandin synthesis and bicarbonate secretion, and critically have direct antimicrobial activity against H. pylori - Sleisenger and Fordtran's Gastrointestinal and Liver Disease, p. 3383; Lippincott Illustrated Reviews: Pharmacology.
• On its own, bismuth has only "modest efficacy" in healing peptic ulcers - the real value is as part of bismuth quadruple therapy (BQT): PPI + bismuth subsalicylate + tetracycline + metronidazole. This is now a preferred first-line regimen for H. pylori eradication, especially given rising clarithromycin resistance, and achieves ~90% eradication rates - Harrison's Principles of Internal Medicine, 22nd ed; Lippincott Pharmacology.
• Safety notes: largely unabsorbed, excreted in feces (causes black stools), but prolonged high-dose use (especially in renal impairment) carries a rare risk of bismuth encephalopathy - Sleisenger and Fordtran's, p. 3383; Goldman-Cecil Medicine.

Bottom line for clinical decision-making

• If the issue is reflux/heartburn symptoms without confirmed H. pylori or an active ulcer needing eradication therapy: alginate (often with an antacid or PPI) is appropriate and low-risk.
• If the issue is a confirmed peptic ulcer, especially with H. pylori infection: bismuth is the more clinically important agent, used within quadruple therapy alongside a PPI and two antibiotics - not as monotherapy. Alginate would not substitute for this because it does nothing to eradicate H. pylori or reliably heal the ulcer itself.
• One interesting historical note: a bismuth-alginate combination preparation has been studied and reportedly showed better local bismuth delivery with reduced systemic absorption/nephrotoxicity risk compared with other bismuth salts, and monotherapy eradication rates of ~40-60% for H. pylori - though this is a niche/patent-level finding, not standard first-line practice.