M-Pidgeot is a good mon, but it suffers from poor coverage. Moltres hits harder since it can run LO/Specs or it can be faster w/ Scarf. Moltres also gets stab on Fire Blast, unlike Pidgeot, so it can really punish steel/electric types where Pidgeot falls short:
252 SpA Pidgeot Heat Wave vs. 252 HP / 0 SpD Assault Vest Eelektross: 86-102 (22.9 - 27.2%) -- 52.8% chance to 4HKO
252 SpA Choice Specs Moltres Fire Blast vs. 252 HP / 0 SpD Assault Vest Eelektross: 211-249 (56.4 - 66.5%) -- guaranteed 2HKO
"Slight" difference in power. Pidgeot is simply easier to wall as well.
252 SpA Pidgeot Hurricane vs. 252 HP / 252+ SpD Lickilicky: 138-163 (32.5 - 38.4%) -- 2.6% chance to 3HKO after Leftovers recovery
252+ SpA Choice Specs Moltres Fire Blast vs. 252 HP / 252+ SpD Lickilicky: 214-253 (50.4 - 59.6%) -- 83.6% chance to 2HKO after Leftovers recovery
I really can't see why you would use M-Pidgeot in RU where so many mons can switch in:
Slowking, Cresselia, Aromatisse, Registeel (lol Heat Wave), Audino, Eelektross, Golbat, Lanturn
And faster electric types - Scarf Rotom-C, Scarf Magneton, and Jolteon
Am I saying M-Pidgeot bad in RU? No, it's still a good mon and have a niche over Moltres since it can't miss and doesn't need a scarf to be fast; but it's just not great in RU. It would need a lot of team support to do very well. NU is definitely capable of handling M-Pidgeot as well, maybe I'm wrong and someone will figure out a core that makes M-Pidgeot broke. As it stands currently, I see it becoming a top tier nu mon.