My morning walk just after the question had been posed prompted me to hypothesise a number of approaches to the question, and I thought it was worth looking at each one in turn and seeing which way it leads...
For a start, I thought, it's possible you'd have some sort of bias that would predispose you towards one or the other.
If you're a fan of a particular musical genre, your mind would, in many cases, already be made up. There aren't too many British zydeco or Cajun acts that you'd be inclined to line up against the Louisiana originals, after all.
Alternatively, someone who's into R&B (and I mean the old style R&B from the Soul era rather than the rap-oriented stuff that has appeared more recently) one could invoke Ben's we're looking at white music escape clause and choose Britain, allowing you to enjoy the likes of Joe Cocker, Steve Winwood and Robert Palmer while still grooving to your Stax/Volt or Motown originals.
But there are other forms of bias. You could be strongly pro- or anti- one side of the Atlantic for political or ideological reasons. I'm not, since my republican anti-monarchist Australian nationalist leanings are nicely balanced by my left wing bias against most of the manifestations of American foreign policy over the period in question and, in particular that self appointed status as the cops of the world (take a bow Phil Ochs)....
Which leads to the question of political songwriters. Any bias there?
You could, for instance, line up the usual suspects and find a pretty solid bias towards North America (the predictable Dylan, the afore-mentioned Phil Ochs and more recent exponents like, say, Steve Earle) but that'd ignore the particularly effective rabble rousing of a Billy Bragg, or the more subtly-stated expressions of protest themes that you'll find in the works of Elvis Costello (Shipbuilding and The Scarlet Tide) or Richard Thompson.
Looking at it in those terms, it comes out as a vote for Britain.