I find that I generally have to try on helmets to determine what will be more comfortable, fit better, etc. Different manufacturers used to use different "models" for head shape, and some helmet brands/models would fit certain head types better than others. For example, some (especially European vendors, it seemed) tended to design mostly for the more "round"/spherical head shape (As looking straight down from above), while quite a few north american heads are more "oval" from this perspective. I found out after some trial and error that I had a "Shoei head", and tended to stick to Shoei helmets ever since as they would fit comfortably, and snugly, as they should. Other brands would create hotspots on my forehead or would be too loose on the sides if I got one large enough to not squeeze my forehead. Of course, if you are talking open face helmets then that makes fitment easier as there are less constraints. I always wear a full face helmet.
Now, several manufacturers have admitted that people's heads do come in different basic shapes, and some are offering models in different conformations to accomodate the round.vs. oval .vs. long head shapes, and also women-specific models too.
After you have found the brand/model type that works well for you, then I'd say ordering off the internet to save some $$ is fine - but if you do it without knowing how the helmet will fit on your head, you might end up with a very ill-fitting helmet that you'll have to try to return.
If (2) different helmets have the same certification ratings, then they had to pass the same levels of testing. Some may have passed by a wider margin, but that doesn't matter - it is only whether or not it met the minimum requirements of the test. Several of SNELL requirements involve multiple hits to the same spot of the helmet. Some people say this is overkill, but I personally think it is a valid requirement. Generally, if you do any track racing you will have to use a SNELL helmet according to the regulations of the track.
Hope this helps.
Mike
