While Second Son has twice the City and a bigger story, there's two things First Light have that makes it better.
1. The powers.
Delsin has access to four power types that all have different utility. Smoke being good for close range brawling, neon being good for sniping, video being good for mid range assault as well as hit and run trickery, and concrete is good for nothing. The crippling issue with Delsin is that he can't freely switch between his powers. In order to switch to a power type, you have to find a source to drain in order to swap. This means if you're not using the best powers for the job, you're in a tough spot unless you can drain some of that power from somewhere.
Fetch, on the other hand, is limited only to neon powers. However, her version of neon powers works a little differently. Fetch contains all the best parts of all of Delsin's powers and as a result, Fetch feels much beter to control and is more fun to play.
2. FL has better endgame content
It's easily the best endgame content the series has seen. At the end of Second Son, you're stuck with whatever optional side content you haven't completed yet. Once you've powered up Delsin and finish the game, there's nothing to test your abilities anymore as all the side content is hardly challenging. At the end of First Light, you have access to survival arenas that push you and your maxed out Fetch to the limit. Once the story is finished, there will always be something to fight with those powers.
I think both games are definitely worth playing! I've played both of them several times. First Light is much, much shorter (and cheaper, price wise) then Second Son, so if you kind of want to just try out the inFAMOUS series to see if you like it, it might be fun to start there. It's also worth noting that the very end cutscene of First Light leads up to one of the beginning cut scenes in Second Son, which is pretty cool I think! But, it doesn't matter which order you play them in!