I was still undecided after reading the existing answers, so I did own research and my thoughts are below. Indeed, considering the specifics in this question, the best choice is likely a lavalier/lapel mic.
Yes, cardioid shotgun mics are good at rejection and in a studio often sound subjectively more natural and clear compared to lavs.
Also, if you are tight on money and want to use one mic to capture both voice and something else, shotgun may be a suitable tradeoff because using a lav to record various ambient sound bites around the location for B-rolls sounds finicky.
Why not shotgun?
Don’t miss that:
Tighter directionality means higher cost and longer length (just physics). If you need portability and are on a budget, this may be important. A cheaper and shorter shotgun is less directional (i.e., just less of a shotgun), and might need to be closer to the subject. Your lens would have to be longer if you don’t want the mic in the frame, and a longer lens is also more tricky to shoot with.
Shotguns reject from sides and rear. This means if you point a shotgun horizontally at the subject (say putting it onto the camera or just holding it off frame without a boom) in a noisy place or outdoors, much of rejection capabilities would be wasted on ground and sky while the mic could still pick up noise from behind your subject. This is why it usually points to the subject’s mouth more or less from above, and if you can’t do that then noise from behind the subject (including possible reflections from a wall) would have to be on your mind, as well as lower frequency reverberations from floor and ceiling indoors (rooms are often smaller vertically than horizontally).
So if you can’t be sure whether your subject will be stationary and always facing the mic, which lens you can get away with, you don’t have someone to follow your subject and don’t want to bring a lot of gear, etc.,—or even if you yourself are the subject and can do everything right, but you still aren’t sure whether the location will always let you sit or stand such that there’s no noise behind you—a shotgun sounds just more error-prone, even if generally sounds better in an ideal setting.
Meanwhile, a lav can reject a lot simply by being close to the mouth and physically shielded from surroundings by subject‘s body—just be mindful of clothing. (If you get cheap lavs, you could mic a person with two of them just in case!) If properly set up, it’ll be easier to get consistent levels, too.