I know the answer is “depends” but…
As with most things I’m on the cusp of sizing charts. I’ve seen a decent BF deal on an Evade 2, but large only. Medium runs 55-59cm, large 58-62cm, and I measure 58.5…
Bar mushroom head, any pro’s or cons of the bigger helmet for racing? fwiw I am building towards an aero position on a new canyon aeroad with the pace drops, so plan to be tucked.
Until you get to the extremes of the new Storm Trooper-styled TT lids, it seems to me that a bigger helmet is going to be slower than a smaller helmet. You are just pushing more mass through the air.
I have always erred towards running the smallest helmet I can. IN your case, I would definitely choose the medium.
And no, I have no data to back up my position other than the old-school “Narrow is aero” 
Kinda thought that myself. And while aero is king, I do hate the mushroom look too. Vivid Red is the bargain colour, which doesn’t go with club kit anyway! I’ll step away from the credit card…
I’m on my 3rd Evade (love them), same head size as you, go with the medium for sure.