I am 1/2 helmet user and did same thing for long trip(4500 mile)to Sturgis etc in 2010. I bought a 3/4 HJC IS-33. It has 2 shields one half shield built in helmet spring loaded and also a outer shield. It worked out great! Harley rebranded this helmet under there name but more expsensive.

 

To get maximum comfort, which ever helmet you decide on, wear ear plugs every day, to protect your hearing.

 

I bought a GMAX GM17 helmet. It is a 3/4 helmet with a flip up face shield and flip tint lens. Both the shield and tinted lens are removable to have an open face helmet. It also has speaker pockets for the headset. The head set mounts on the lower edge of the helmet for a secure connection.

The liner is removable for cleaning and has a number of vents for air flow.

I put the face shield and tinted lens on for a 3,200 mile ride and haven't taken it off since.

I've bought a second to replace this one when it wears out.

http://www.gmaxhelmet.com/GM17SPS-SO.html

 

I have both a FF and a 3/4. In the summer I prefer the 3/4 with Wiley X goggles. I did a 4K mile trip this summer with this setup. Either way using a helmet liner keeps you much more comfortable.
I like this one, about $12 on line.
http://www.sweatvac.com/ventilator-c...php?page_id=61

 

How far west are you planning?

 

I wear a full face on everything except dirt bike and big Harley. But I have a big barn door on my big harleys with half helmet. Use a Microfleece Balaclava if it is cold. Full face helmets are great for wind protection and comfort in the wind/weather.

 

I run a Scorpion 3/4 with a removable visor that is replaced by a flip up shield. the shield fits perfectly in saddlebag without taking up too much room (slides down the rear of the saddlebag and curves over the top of other gear in bag). havent' had a need for full face since purchased - like it so much, purchased a second so one is wired with headset and one isn't depending on riding plans.

 

Fulmer full face modular on long trips, Fulmer 3/4 for everyday riding. Modular helmets are the way to go on long trips.

 

I run a modular flip front HJC w/ IMC headset. Plenty of air when I want it and keeps most of the weather out when needed.

 

Can you ride with a modular helmet open ???