I've rarely used a helmet in a car, but worn them regularly on motorbikes for over 20 years.
Rule 1 - wear one that fits your head. It's not for comfort reasons (though there is that), but because you want a snug fit all over your bonce. That way, the helmet starts to absorb any impact from the moment of impact.
Rule 2 - wear the most expensive helmet you can afford (unless your head can be replaced cheaply). In my experience, you really do get what you pay for. I mean in terms of protection, of course, but also durability, visor quality, and ease of replacement for cheek pads, vents etc. SHARP (Safety Helmet Assessment and Rating Programme) is a bit like NCAP for crash helmets, but it's controversial. Use it as a guide, by all means, but refer to rule 1 first!
The D ring fastener is a good idea. Yes, it's fiddly until you get used to it (which you will), but it also guarantees that it will be done up correctly. I've seen a buckle-type helmet that was "done up", but which was so loose that it came off on impact - really not nice.