JTL:
Do a search for "strings" within this forum, and you'll get some opinions. Or do a search for "bluegrass" in the string forum
I've been around novice players to professional recording bassists. Preferences are usually player dependent and range from metal to synthetic to nylon to gut (even a few with weedwackers, but those are not my cuppa). Go to some festivals and play a few basses and get a feel for what's available. It really boils down your hands, your bass, and the sound you want.
Having said that, if you still can't decide, my opinion is that the path of least resistance is Spirocore Mittels. Plenty of bluegrassers play them, they're punchy, they cut through a mix well, they stay in tune, and they will last a very long time. I prefer gut, but whenever I cheat, it's with a set of very old Spirocores.
Good luck,
Thomas