Haven't played with Cassandra for ages, so I can't really say if anything's changed, but I'd say that Randy is generally easier. Choosing a storyteller has also other effects, than number of raids or other challenges, so it's not just a difficulty option, but a choice which meaningfulness to you depends on your play style and goals. For example I never build space ships, instead I wanna build a large city like colony.
With Randy on you can have really hard time for a while sometimes, and like JohnLG says long periods with nothing major happening. People argue Randy can be boring, but I have no hard time finding things to do when it's relatively peaceful. Its possible to reach 50 or so colonists before Randy gets really nasty. Also, as I pretty much accept anyone who joins, and recruit the ones I like, there can be some challenge in management just because of variety in colonists temperaments etc.
Cassandra is (or was) like when everytime I reached a population cap, 16 colonists or so, she started to tighten the screw until someone died. This could go on and ultimately endanger the whole colony - if I played well and no colonists died, I started to think about sacrificing someone just to get a moment of rest. Got tired to continous pressure of about similar intensity. Also I needed people to have stuff I wanted done, can't really have a large city like colony with 16 colonists - so I chose Randy and have then stayed in that choice.