Aesthetically, Pidgey and its evolved forms rule the day but what hurts Pidgey is that it doesn't learn its first Flying move until level 28 with Wing Attack. Spearow, on the other hand, knows Peck from the start and learns one of the better Flying Type moves in Drill Peck at level 29. However, I've always been a fan of Sand Attack and Quick Attack which Pidgey learns relatively early(level 5 and level 12 respectively) so although it doesn't know Flying Type moves until much later, its utility with that Sand Attack makes it a very versatile Pokemon early on.
In terms of speed and power, Spearow also has the edge but Pidgey is a bit more balanced with higher defense. If you get the Fly HM on Pidgey, then Pidgey isn't that bad of a choice with Fly and Wing Attack to go along with Quick Attack and Sand Attack. I always catch both and I always utilize both since they have their uses. If I truly had to pick though, I'd probably go with Pidgey since what makes Spearow can also be found on Doduo(and Doduo is better too). So my choice is Pidgey(Pidgeot) over Spearow(Fearow). If I added Doduo into the mix... it'd be a tough call but I'd probably still go with Pidgey.