Both are based on a transforming vehicle-to-robot toyline by Takara Tomy, and both have a similar initial premise. A young boy gets involved in the organization his father works for and becomes a driver of the titular robot due to high compatibility when attempts to get an adult driver failed. From there, they protect the peace from a mysterious enemy while keeping their new position a secret.
Main difference is that Shinkalion has a lot more plot and a much higher budget. Drive Head actually has an interesting plot too, but unfortunately has severe pacing issues, so it follows more of a monster-of-the-week format for the most part until they get there. Still, they have a similar feel, and even canonically crossover in Drive Head's movie, so it's worthwhile to watch both.