either has ups or downs really
helicoil you can keep the original stud size which goves a lil wiggle room on the headers since nothing ever fits perfect the lil movement is useful at times
drill and tap for larger stud well you have to drill the head not hit oil or coolant galleys,make sure its tapped straight then drill the header manifold to also accept the larger stud preferably a touch larger to give that wiggle room back ,providing the header has the clearance for the larger planned stud and nut
all things im sure you've thought of but just pointing it out incase anyone reads this thinking there isn't much to it lol