Lv50 Lanturn's Thunderbolt (15 IV, Neutral, 128 EV) vs. Lv53 Walrein (25 IV, Neutral, 0 EV): 102 - 160 (52%-61.2%)
Lv50 Starmie's Thunderbolt (same conditions) vs. same Walrein: 82 - 98 (41.8 - 50%)
Glacia's Walrein may not necessarily have 25 IVs in Special Defense, but Lanturn has a better chance of 2HKO on Walrein than Starmie. Starmie is having roughly 70% of Lanturn's firepower with Tbolt. This is definitely not a minute difference. Granted, I'm not really sure how high-levelled would a typical party be at E4 (the only thing i can ever remember is smashing them with an overlevelled blaziken) but if anything this is probably it.
Honestly both are interchangeable imo.
and uh if you'd
Lv53 Flygon's Earthquake (25 IV, neutral, 0 EV) on Lv52 Lanturn (15 IV, neutral, 128 EV): 152 - 180 (73.1% - 86.5%) Lanturn is really only OHKO'd if he has like, no defense EVs (which isn't likely seeing as you probably use him to murder geodudes in victory road or something)