Grassfed beef has marginally more omega 3 to balance the omega 6. But the most exquisitely pasture-reared pork, or organic chicken, has an order of magnitude (or more!) omega 6 in it than the most CAFO nightmare of beef.
In other words, don’t sweat it. Paleo sentimentality has clouded the more important nutritive issues here.
I tend to opt for grassfed meat primarily because of animal compassion concerns - as with people, pumping them full of grains is nasty for their well-being. Also, most British beef is largely grassfed, so it’s not difficult; however, if you had to choose between US “Walmart beef” and “Wholefoods organic chicken”, then, for nutrition profile, you’d probably do better opting for more of the beef. Even the Walmart beef.