Whilst i understand the desire to ascertain which is the "best" groundschool it is largely a fruitless endeavour.
Both OATS and BGS run excellent courses with which I would not have a moment hesitation in placing a recommendation.
Bigger deciding factors would be location, price or 'the vibe' you get when you walk in the door.
The same goes for nearly every other well established groundschool in the UK.
There are no definitive statistics on groundschool performance collated in the same way by an independent source such as the CAA. In the absence of this all you have is student claims followed in this case by staff/owner claims.
Whilst everybody is posting with the best of intentions I think this thread is becoming a little pointless.
Carry on if you want though.
Cheers,
WWW