A helmet that is the right size should feel snug all the way round with no single pressure point, and it should not shift when you turn your head firmly from side to side. A helmet that moves independently of your head on a shake test is not the right fit, regardless of what the size label says.
To compare our full face and open face ranges side by side, see https://youngstersprimer.a2hosted.com/index.php/Getting_The_Fit_Right:_A_Guide_To_Helmet_Sizing.
A helmet is the most visible piece of riding gear, but it works as part of a wider kit rather than on its own. Gloves protect hands in a low speed drop, which is one of the most common types of fall, and a good pair balances grip and feel with real abrasion resistance.
None of this needs to be bought at once. Building a complete kit over time, prioritising the areas most exposed in a typical fall, is a realistic way to get there.
Any impact, including a helmet dropped from a moderate height, can compress the inner liner in ways that are not visible from outside. A helmet is designed to absorb one significant impact by sacrificing that inner material, so a helmet involved in a crash should always be replaced, not inspected and reused.
Neither format is a universal answer. Weather, speed, journey length and personal comfort all play a part, and many riders end up owning one of each for different trips.