@Fireball - Helmet comfort is largely about head shape and matching your head shape to the helmet you wear. With half shells, it didn't matter much. With a full face or modular, it matters a lot. Most LD riders prefer modular helmets, as igneouss mentions above. I also prefer modular helmets. I started LD riding with an excellent fitting Arai FF, and quickly realized it's limitations on long rides.
Go to a dealer that stocks a lot of helmets and talk to someone about your head shape. See if you can determine if you're round, oval or mid oval. Those are the most common shapes. If you're European decent, head shape tends towards oval to long oval. Shoei helmets are mostly round, but they do offer some mid oval styles. Many of the manufacturers offer both, some only offer one shape, or just generically put everything as mid oval.
In the store, try on some different brands and walk around for a while with it on in the store. If you're an oval guy, you'll get a hot spot on your forehead pretty quickly if you're wearing a round helmet. If you're a round guy, your ears will get squished, assuming you're wearing the same size helmets.
Be careful not to cheat the issue by wearing a larger helmet that's not your shape. As the helmet breaks in, that ends up being worse. All new helmets will be on the tight side of fitment. Think of it as a performance fit and it will break in some over time/miles.
If mid oval, check out LS2 helmets for very competitive prices and good quality. Lighter is nice on long rides and the Carbon Fiber helmets will usually be the lightest in their different classes. Modulars tend to weigh a little more than full face versions, but the trade off in the ability to drink, eat while riding and hold a conversation when stopped is worth it.
Once you know your size and head shape, you can do a lot of research online in pretty short order and have a high degree of success on ordering a helmet w/o trying it on first.
I like venting front and back for flow through.
Pinlock ready visors. Drop down sun shields so you can just move the sun shield down w/o changing glasses or main visors.
It sounds like this will be a new experience for you, (wearing a full face or modular helmet). If so, expect it to take some time and rides getting used to it. Most of us wear ear plugs or in ear monitors as well. Noise is a stressor and will cause greater fatigue, not to mention permanent hearing loss. When you're riding ~2300 miles in 50 hours, it makes a difference. Find a Safety Supply store and buy a bunch of different styles of foam earplugs if this is new to you. See what works for YOU. If your ear canals get sore, a tiny amount of Neosporin helps.