So my question is, why not a bolt.
Aesthetics mostly. A nut and bolt are pretty rough looking compared to just epoxying the joint or even using pop rivets.
If it were me, I'd use two or three aluminum pop rivets. They will be plenty strong to hold the comptip in during trailering and you won't have any issues with galvanic corrosion of the mast (as compared to using stainless steel or monel rivets). They are also super easy to drill out in the event that you want to remove the comptip at some point in time.
I would hook up the downhaul or mainsheet blocks to the main halyard and use them to compress the comptip into the lower mast section, then drill the holes and install the rivets. This way you reduce the chance of the comptip/rivets loosening up.