Festival of Blood is more like an addon for fans, at least from what I´ve seen of it. Didn´t have a PS3 when it came out.
Infamous 1 and 2 are some of the best games on the system. You should play them.
Both infamous games are on my "underrated open world games" list, together with Red Faction Guerilla.. they need more love from da people!
I found the first game to be underwhelming and quite tame but also, as you say, decent. I haven't played anything past it. Festival of Blood seemed kind of neat though. It almost made me want to give it a shot. Almost.
So maybe you fall into the camp that I and others here seem to be in with regards to these games and just find them to be okay. If Infamous 2 is still one of the PlayStation plus freebies maybe that would be the way to go? Seems it still is. And now so is Vanquish... I might sign up for a 3 month spell, myself.