1. Recovery Speed & Core Application Focus: The MUR120RLG is a fast recovery diode (trr=35ns) from ON Semiconductor's SWITCHMODE™ series, specifically optimized for high-frequency switching circuits (e.g., flyback snubbing, freewheeling, and rectification in SMPS). The 1N3611 is a standard recovery diode (trr>500ns), suitable for line-frequency (50/60Hz) rectification or low-frequency circuits insensitive to switching speed. Using the 1N3611 in high-frequency circuits like SMPS will lead to severe reverse recovery losses and voltage spikes, potentially causing drastic efficiency drops, device overheating, or even failure. This represents a fundamental performance mismatch. 2. Forward Voltage Drop (Vf) & Conduction Loss: The MUR120RLG features a lower typical Vf (875mV @1A). The 1N3611 has a higher typical Vf (1.1V @1A). At identical operating currents, the 1N3611 exhibits higher conduction loss, resulting in increased heat generation and lower system efficiency. The MUR120RLG holds an advantage in efficiency. 3. Manufacturer Series & Market Positioning: The MUR120RLG is explicitly marked as part of the "SWITCHMODE™" series, indicating its design optimization for high-frequency switch-mode power supplies. The 1N3611 carries no special series designation, identifying it as a general-purpose standard recovery diode. This confirms the performance positioning outlined above. The MUR120RLG is a specialized component for modern, high-efficiency power supplies, whereas the 1N3611 is a legacy general-purpose device. Additional Important Note: The price you provided for the 1N3611 ($4.42) appears significantly anomalous and is far above the market rate for comparable standard recovery diodes (typically under $0.1). This pricing may invalidate any cost-based substitution assessment. It is strongly advised to verify the source and quoted price.