I will say as far as discriminating, browns can be as aggressive and opportunistic as brookies in less fertile, less fished streams. WHEN THEY'RE FEEDING.
But. You'll notice in streams that have both. The brookies don't really shut off as easily as the browns. If the water is real low, or it's midday, or whatever, all you'll catch is brookies, it can be like the browns don't exist. Hit that same water when it's up a little, or off color, or morning/evening, and the brookie/brown ratio is WAAAAYYY different.
There's one well known stream, where it's nearly all browns up to a waterfall, nearly all brookies above. Above the falls is good for a fairly reliable 50-100 brookies. Bad conditions is like 50, good is like 100. If you spend a day below the falls? Hit it right and you can be in that 100 range. Hit it wrong and it's zero point zero, like Bluto's grade point average, and you'd swear there's not a fish in it.