I haven't had occasion to splice many masts, but I've certainly had to deal with many many fasteners on masts, broken, mangles, frozen, or otherwise!
Based on nothing other than personal observation/experience, panheads are weaker than tapered (ie., needing to be countersunk) screw heads, on account of the more abrupt transition between the shank diameter and head diameter. Additionally, countersunk screws show more metal-to-metal surface area, equating to a better distribution of force and less chance of loosening. Panhead screws seat best on a flat surface, whereas a countersunk hole can be applied to most any profile.
Agreeing with Douglas, ovalhead screws have more "meat", which can certainly make them easier to remove when needed! Here in the islands, ovalheads are harder to source, but I can't see how the barely-protruding head has any other drawbacks, unless the splice is at some area where halyards.sails are likely to chafe.
While a self-tapping screw will "draw" pieces together, I can't help but wonder if greater strength and serviceability might be had from tapped-in machine-screws, provided the internal sleeve fits closely enough that "drawing in" is not required.
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