Well... Best I can remember, helmets are tested to an ANSI-standard, and all helmets with that label are supposed to provide about equal protection. There's a fairly rare Snell standard too, which is a bit harder. But I haven't seen a Snell-approved helmet in years. Either way, to get any value out of a helmet, you have to use it. So you do need to find one that you like enough to actually put on. After that, it's of course a good idea if it's a decent fit, and doesn't rattle around on your head. Even if there are straps, inserts and whatnot, it's a good idea if the contours of the helmet is a reasonably good match to your skull. Some are more oval, others are rounder. "MTB" helmets tend to have visors, and a mesh across the front vents. Offers a bit of protection from poking a branch or something through the vents and into your scalp. They'll deflect the occasional bug too. Then there are the full-face MTB DH helmets too, these days often backed by a collar for the serious riders. Some would consider them OTT for casual/commuter use, but your first facial fracture tend to make you reconsider. "Road" helmets tend not to have visors or mesh but will otherwise be pretty much the same - unless you go to TT helmets. A fair amount of people these days use what's originally skateboard/inline helmets, identifiable by a rounder shape and harder outer shell. Often a more discrete color scheme as well. I think they're tested to a slightly slacker standard, but opposed to a bike helmet, they're intended to offer decent protection for several impacts. Bike helmets will take some knocking around, but are meant to be discarded after any serious hits. What's actually "best" between these two is probably impossible to tell, and likely to hinge on very minor differences in how and what your head strikes. A "clean" fall onto the rear of your head, then a bike helmet will have more impact absorbtion. A fall onto the side of your head, then a skate helmet might do better on account of its lower sides. Personally, apart from fit I look for helmets that have removable straps. If you intend to ride until sweaty, the straps will get a bit mucky after awhile. Some will give them a rinse while attached to the helmet but I prefer to remove them and send them through the washer. Now, some may say that if you haven't needed a helmet in 11 years, what makes you think you'll need one in the future? While there is some merit to that thought, it only takes something as silly and undramatic as a slow sideways topple to split an eyebrow. And for what they cost, and how easy they are to wear, I'll happily use a helmet even if that is all it's ever going to save me from.
