Magma Storm in Sun 150 BP+1/8th HP per turn
75% Accuracy
Fire Blast in Sun 165 BP @85% Accuracy (10% Burn chance for residual damage, mostly negligible here)
The damage is close to comparable, but if you hit a burn, I think Fire Blast ends up doing more damage. Fire Blast will hit a lot more often, that's just numbers, so by that metric it will out damage Magma Storm in all likelihood even if their regenerator core is switching as you're likely hitting for +50% and hazards each turn. Scarfed Magma Storm does entirely eliminate the problem but will be less reliable.
252+ SpA Heatran Magma Storm vs. 248 HP / 92 SpD Tornadus-Therian in Sun: 267-315 (73.9 - 87.2%) -- guaranteed 2HKO after trapping damage
252+ SpA Heatran Fire Blast vs. 248 HP / 92 SpD Tornadus-Therian in Sun: 294-346 (81.4 - 95.8%) -- guaranteed 2HKO
252+ SpA Heatran Magma Storm vs. 252 HP / 4 SpD Slowbro in Sun: 162-191 (41.1 - 48.4%) -- 72.7% chance to 2HKO after trapping damage
252+ SpA Heatran Fire Blast vs. 252 HP / 4 SpD Slowbro in Sun: 177-209 (44.9 - 53%) -- 29.7% chance to 2HKO
252+ SpA Heatran Magma Storm vs. 252 HP / 4 SpD Toxapex in Sun: 99-117 (32.5 - 38.4%) -- 8.6% chance to 2HKO after Stealth Rock and trapping damage
252+ SpA Heatran Fire Blast vs. 252 HP / 4 SpD Toxapex in Sun: 109-129 (35.8 - 42.4%) -- guaranteed 3HKO after Stealth Rock
(Lol, good luck with this one)
252+ SpA Heatran Magma Storm vs. 252 HP / 176+ SpD Assault Vest Slowking-Galar in Sun: 129-153 (32.7 - 38.8%) -- 6.6% chance to 2HKO after Stealth Rock and trapping damage
252+ SpA Heatran Fire Blast vs. 252 HP / 176+ SpD Assault Vest Slowking-Galar in Sun: 142-168 (36 - 42.6%) -- guaranteed 3HKO after Stealth Rock
So for Pex it's kind of meh. It could be packing Shed Shell, and without Taunt it can just Recover anyways. If Glowking is AV you can trap it, but if it's any other set it could have access to Slack Off. If Slowbro is the Colbur variant and you have rocks your rolls to 2hko before it can Slack Off are much better. Torn gets blown up by both, and Modest Scarf still outruns 168 Timid Torn. Then there's this...
252+ SpA Heatran Eruption (150 BP) vs. 248 HP / 92 SpD Tornadus-Therian in Sun: 399-469 (110.5 - 129.9%) -- guaranteed OHKO
252+ SpA Heatran Eruption (150 BP) vs. 252 HP / 4 SpD Slowbro in Sun: 242-285 (61.4 - 72.3%) -- guaranteed 2HKO
252+ SpA Heatran Eruption (150 BP) vs. 252 HP / 4 SpD Toxapex in Sun: 148-175 (48.6 - 57.5%) -- guaranteed 2HKO after Stealth Rock
252+ SpA Heatran Eruption (150 BP) vs. 252 HP / 176+ SpD Assault Vest Slowking-Galar: 129-153 (32.7 - 38.8%) -- guaranteed 3HKO after Stealth Rock
That's probably the button you want to be clicking most of the time.