If you're concerned about the mushroom look it's really just a case of trying a few on and seeing what they look like on you, some will look sleeker thanks to a smaller shell, some will always look like a mushroom. And it's not always make-dependent... sometimes if you're at the low end of "Large" say, even the raciest of helmets might still look like a mushroom. It usually looks a bit neater if you're toward the higher end of a range, so for example where one brand might have a range of 55-58cm in Medium and 57-60cm in Large, and your noggin is 57cm, then the medium will usually look better.
Bear in mind when a helmet shell is more snug you might potentially lose the flexibility of putting a hat under it in bad weather though. Personally I prefer to size down, and I like the neat look of POC helmets, but everyone has their own preference.
Fit is the really, really key element though, make sure it fits properly around the right part of your head and is comfortable.