It's a question I've been facing a lot recently as I'm refusing to change to long trousers until I absolutely have to, and dog-walking paths are at the beginning-to-be-overgrown stage. And there's no question in my mind: it has to be brambles every time.
Brambles are very up-front and aggressive. 'Don't mess with me, baby!' They reach out, grab and scram you viciously, making you scream, 'Aaaaaaaaaahhhhhh!!!' Nettles on the other hand lull you into a false sense of security as they waft daintily in the breeze before brushing casually against your leg. 'Oh, ah, ee, oh, oh, ah, ee, ee, ee.' The stinging is underhand and the effect gradually works its way throughout your body, shiver-inducingly. For ages you'll be walking along moaning softly to yourself.
And they don't leave any trace that's how sneaky nettles are. Brambles at least leave a good manly scar that you can take pride in. You definitely know you've been brambled. But nettles, huh.