Based on research/reading, I'm leaning towards the chromes (medium).
Question: Are the chromes all the same length, so the silk windings all start at the same distance from the nut?
The La Bellas I used before have a couple of different lengths in a set, which, depending on where your tuners are, can result in the windings (not just the silk portion) going around the tuning pegs. La Bella emphasizes to not let windings go around the pegs, saying the string will break.
I've been dealing with the raspy piano sound of rounds on my bass, thinking flats were useless for slap, but in hearing several YouTube vids, I actually prefer the flats slap sound. I realize Marcus Miller, my favorite player, would give me the "stink-eye" for going with flats, but I like the old-style sound.
This article suggests that Chromes are probably the top pick if you want to slap on flats: Flatwound Roundtable - Guitar Pickups, Bass Pickups, Pedals
I'd mostly play finger and pick on them, but I'd like to add some slap to my "bag of tricks".
I'm no slap genius, but I like to do it some; and chromes worked find for me. I tried the mediums, but the seemed to heavy to me and I like the lighter gauge Chromes better.