I know the counter-argument. Voldemort was called the most powerful dark wizard in history! But that is from Britain's perspective. Grindelwald was never powerful in Britain, wheras Voldemort was, and much more recently. Voldemort also did more evil things. The one person that could be trusted on the matter said Voldemort is the most dangerous. Most DANGEROUS, not powerful. Grindelwald was not as extreme and evil, but even Dumbeldore said Grindelwald was roughly his equel in power, while Voldemort was known to everyone as being far less powerful. Dumbeldore defeated Voldemort in like 20 seconds at the ministry, forcing Voldemort to retreat. Grindelwald was powerful in almost all of Europe, conquering multiple wizarding countries, whereas Voldemort never had the power to leave Britain and extend his wrath! Grindelwald was also wiser and more knowledgable, and while Voldemort always had to fight through subterfuge and spies, Grindelwald was powerful enough to have straight out battles and conquer and stuff.
Nearly all the books make out Voldemort to be the greatest dark wizard of all time. Grindelwald only came to power after he stole the elder wand and any success he had outside of Britain was only because Dumbledore refused to face him (for fear of finding out who had killed his sister) if dumbledore or harry hadn't been around Voldemort would have won. During the first wizarding war Dumbledore and the ministry worked against Voldemort for 10 years and from the sounds of it Voldemort was going to win if not for harry. The duel between Dumbledore and Grindelwald is never given any real detail in the book so we don't know how well either wizard performed against each other and for all we know Dumbledore at the time was not as powerful as when he fought voldemort with the elder wand. So even with the elder wand Grindelwald was still defeated and captured by Dumbledore. Saying Voldemort only got as far as he did because of his horcruxes is an unfair assessment because horcruxes only matter if your opponent strikes a killing blow so even if Gridelwald had horcruxes and the elder wand whe he fought Dumbledore he still would have lost because dumbledore could beat him without killing him. So my opinion is horcruxes don't matter in a duel between them and even if Grindelwald has the elder wand I still think Voldemort is the more powerful wizard so in the end I say voldemort would win though it would be a close fight.