it's hard to compare.
that relatively rare child who's the flat out brightest in their primary school class (or, especially, their year, if there's a multi form intake), provided they've been paying enough attention in class to be strong in the fundaments of literacy and numeracy, will usually be able to get WT scores of around 130 or so per paper, maybe a little better or worse, without having ever clapped eyes on a VR or NVR test. i suppose I probably only think that the difference between the best and worst possible prep for those kinds of tests (short of kids turning up for a test tired or hungry or whatever, which of course does happen) can only be worth, what, not all much more than a dozen points?
but even the brightest child across a number of schools, in a whole town or whatever, wouldn't be able to pass a grammar school test without prep, certainly not the maths bit, there'd just be too much unfamiliar content. a really, really bright kid who was also a really avid reader would probably be able to have a good go at the English bit without any prep.
i think for an only very moderately bright but exceptionally well prepped kid the GS test might be easier, other than perhaps for the very most super selective schools with the very highest pass marks. but in the main I'd say that the WT is easier, though I certainly do know kids who didn't pass WT but did get a Sutton grammar place.