Depends on what you're looking for.
Here's an extremely comprehensive SA post on how the storytellers work. If you've got an account there or they aren't running a membership drive, refer to that instead of this short summary of it.
Cassandra and Phoebe have 10 bins of events to pull from, they start at bin 1, and go up a bin with every raid. This is hardcoded. Cassandra delivers two raids per month, Phoebe delivers one raid per month. They never go down bins, and never look at the state of your colony to decide what's appropriate. Randy is not hardcoded to send anything at you. He'll look at your colony, pick from the state of it events that will mess with you regardless of the bin, and throw something at you. If the comparative value of the event is low and your colony is large and prosperous, he can throw additional low-bin events out in rapid fashion.
The drawback is that he will sometimes toss a bunch of well-equipped raiders at you at the start instead of tribals, or some mechanoids, or something else that you just aren't prepared to deal with due to the low time you've spent in a colony. The upside is that he won't get stuck at bin 10 after a few months and just spam 50+ man raids, psychic ships, and generally all the horrible nastiness that Cassandra and Phoebe get stuck at. If you don't mind taking some chances in the early game, then you can have a long term late game with Randy, without necessarily having to use the more gamey tactics necessary for managing the highest grade of horrible that the storyteller can dish out.
Personally I've been using Randy, and I have no real complaints on it. Granted, I turn off the more arbitrary bullshit like solar flares and random electrical shorts, but that's just a personal QOL thing.
The difficulty level is an entirely separate thing, the mechanics are discussed quite well
here. I've gone for 'some challenge' and above, but I came in having read some starting player guides and having previous experience with builders of this nature in Towns.
EDIT: One
starting guide while I'm at it.