The fit is probably more important than the type of helmet... we all have different shaped heads, and you'll find some brands will suit you better than others... so try before you buy and try lots of brands.
As far as types are concerned, riders in my side of the woods are more accepting these days. Space-ranger XC types, or pot-lid skateboardy BMX types... no body seems to care what other people wear anymore...
I'll say just choose what you're comfortable with the most... the "usual" XC type is light weight with lots of holes, so if you plan on riding long hours and long distances you're best to stick to that.
The skateboardy helmet is sturdier, heavier and hotter (not many vents), but it wraps around your head more and offers better protection, so if you plan to do a lot of jumping and extreme stuff...
Besides these two main styles, there's actually a third style emerging which is sorta a mutant of the two... lightweight with more-ish vents, but with better "lower-back of the head" protection... you might want to check those out too...
