This prospective randomized study compares the clinical and radiological long-term outcomes of single-level anterior lumbar interbody fusion (ALIF) and total disc arthroplasty (TDA). Patients with symptomatic single-level degenerative disc disease (DDD) at L4/5 or L5/S1 were randomly assigned to groups ALIF or TDA. Clinical evaluations using the Oswestry Disability Index (ODI) and visual analogue scale (VAS) for pain were conducted preoperatively, at three, 12, and 24 months, and after a mean follow-up of 14 years (12.2 to 15.9). Radiological assessments included radiographs in two planes and flexion-extension views. Additionally, CT was performed in the ALIF group to evaluate fusion after 24 months. Complications and patient satisfaction were recorded. Outcomes were analyzed for the entire cohort and by spinal segment. Of the 120 patients included (60 per group), 28 were lost to follow-up, including three excluded because of revision surgery. In the remaining patients, significant improvements in ODI and VAS were seen over time (all p < 0.001). Clinical scores had declined slightly by final follow-up but remained better than the preoperative levels. No significant overall differences were found between ALIF and TDA. However, subgroup analysis revealed that ALIF outperformed TDA at L5/S1 (ODI posthoc test at final follow-up p = 0.005): outcomes were comparable at L4/5. Both ALIF and TDA are safe and effective methods of treating single-level DDD. ALIF is preferable at L5/S1 due to biomechanical factors, such as variability in the centre of rotation and sagittal profile types, which have a negative impact on the outcomes of TDA at this level. Conversely, at L4/5, both procedures give comparable results. These findings emphasize the importance of considering segment-specific anatomical and biomechanical factors in surgical decision-making for DDD.