Well my advice is don't order over the net...try them on before you buy. I wanted (so I thought) the Red HiFi II (which is what I assume you mean by Burton HiFi), I tried on the large which should have been my size according to their charts and found that it was too short for my head front to back. Like it is for a head rounder than my own. So I figured I would have them (my local shop) order the XL and it would be fine since it has the air pump for helping the fit. When the XL came in it fit but was like I was swimming in it...and it looked huge on my head so I didn't take it (thankfully they were cool with that, given the money I spend in there they should be though LOL). I tried on several Giro helmets which fit my head shape better but they didn't have a style I liked and then I tried on the Bern helmets. I think the Bern run smaller than the Red (Burton) and XL would have been the right size but they too fit me weird...it was hard to put on but once I got it on it seemed to fit well. I was a bit uncomfortable with the issue I had getting it on and was concerned that wearing for a long time (like a day of riding) might be an issue. In the end I went with a Giro I had to get at another store, simply because they had it in stock.
Moral of the story is try them on, many times helmets from different companies fit differently and you need to find the one that is right for your head. I had gone in to spend more money than I came out having spent in the end because the "higher end" helmets I went in to look at didn't work for me.