Honestly both routes have their ups and downs.
Slowly Growing Sig.
+ Sense of Progression
+ Everyone starts with some sort of Sig. move no matter how strong
- Lack/less of an impact when moves progresses
- Makes having a Sig. move less special (If everyone, even less experienced pokemon, have these types of moves it makes them seem less of what they are
This video helps explains)
All At Once Sig.
+ The move is nice and powerful at the beginning
+ Gives a sense of strength between PC's and NPC's (Less strong pokes won't have one and strong ones will)
+ Makes obtaining a sig. move more special
- Large power gaps. (It's like a tackle vs. a giga impact.)
(Feel free to add to the list if I've missed something or post any conflicting thoughts)
Honestly, thinking about it more I feel that going the, for a lack of a better term, "All at once Sig." method is the better choice