I'd say bass for the most part. Most kicks sit around 60-80Hz but a bass guitar (and synth obviously) can go way lower (40Hz for the open E-string or so? Closer to 30 if you have a 5 string bass with a B string I think).
With electronic, 808-like kicks it's a different story, you could have a good thump at 40Hz or even lower, like always it "depends"
Generally speaking I let the bass decide - esp if 5 string or 'sub bass' - and most of the time the bass ends up lower. And if the bass is, for example, pulsing 8ths, I might sidechain it to the kik so that it ducks when the kik hits (i.e., since otherwise the kik may not have enough punch).
I often spend more time on kik and bass than anything else in a mix . . .
it all depends on the style of music and the instruments used. For reggae the bass must go as low as possible, and the kick often get hpf-tered till even 100hz, for mainstream dance the kick must go lower than the bass because the hit gives the groove ppl are dancing to, and everything in between is also possible. They even often duck the bass by the impact of the kick trough sidechaining (lowend kick in the sidechain to bass compressor)