Depends on what you want.
1. Phoebe's best for building and development, as you've already noticed. She's heavily weighted towards minor threats and has minimum length of time between major threats of just short of one full season (12.5 days default, adjusted by difficulty). Her threat cycle is also the longest, which as far as I can tell also plays into average times between threats.
2. Cassie's for a steady increase in difficulty, apparently ramping up as you go. She gives about a 3:1 ratio of minor to major threats with a bare minimum of 2.1 days between major threats. I'm not sure if her timer resets if you switch to her or if she goes based on time since landing, but I'd assume the latter absent evidence. I don't pretend to understand the storyteller code, so I don't even see where the difficulty ramp is altered based on time in the storyteller XMLs. EDIT: Hmmm, it might actually be the threatCycle. Maybe.
3. Randy's all over the place, apropos of the name. He'll on average fire at least 1-2 threats per day with an even chance whether it's a small threat, a big threat, some friendly arrival, or orbital traders, and guarantee at least one event per day. He'll also guarantee at least one major threat every 13 days by default, unlike the others who lack a maximum time interval for any sort of threats. One other unique trait he has is that, while the other storytellers won't give you additional settlers above a population of 18, his critical population threshold is set to a surprising 50. It's typically not a big thing, but it's worth noting if you want a *really* big settlement.
I tend to play Randy, personally, but I know some people swear by Cassie due to Randy's erratic nature (there's nothing in his code stopping him from dropping two major threats on you in a single day, but he might also hand you two free migrants back to back instead), and those who are just in it to manage a colony will go with Phoebe. I suppose it depends on what you're after.