Introduction: Somatostatin analogs (SSA) prolong progression-free survival (PFS) in patients with well-differentiated gastroenteropancreatic neuroendocrine neoplasms (GEP-NENs). However, the eligibility criteria in randomized clinical trials (RCTs) have been restricted, which contrasts with the vast heterogeneity found in NENs. Methods: We identified patients with well-differentiated (Ki-67% ≤20%), metastatic GEP-NENs treated in first line with SSA monotherapy from the Spanish R-GETNE registry. The therapeutic effect was evaluated using a Bayesian Cox model. The objective was to compare survival-based outcomes from real-world clinical practice versus RCTs. Results: The dataset contained 535 patients with a median age of 62 years (range: 26–89). The median Ki-67% was 4 (range: 0–20). The most common primary tumor sites were as follows: midgut, 46%; pancreas, 34%; unknown primary, 10%; and colorectal, 10%. Half of the patients received octreotide LAR (n = 266) and half, lanreotide autogel (n = 269). The median PFS was 28.0 months (95% CI: 22.1–32.0) for octreotide versus 30.1 months (95% CI: 23.1–38.0) for lanreotide. The overall hazard ratio for lanreotide versus octreotide was 0.90 (95% credible interval: 0.71–1.12). The probability of effect sizes >30% with lanreotide versus octreotide was 2 and 6% for midgut and foregut NENs, respectively. Conclusion: Our study evaluated the external validity of RCTs examining SSAs in the real world, as well as the main effect-modifying factors (progression status, symptoms, tumor site, specific metastases, and analytical data). Our results indicate that both octreotide LAR and lanreotide autogel had a similar effect on PFS. Consequently, both represent valid alternatives in patients with well-differentiated, metastatic GEP-NENs.