So many people have asked me about how they compare that I gave it a bit of thought. First I should say that I loved both and appreciate the difference in races in all sorts of ways. Their different terrain, scenery, people, atmosphere, scale and quirks are what make them all fun.
The GRP is much bigger, more commercial and set in a bigger town. It attracts more runners and bigger sponsors. The Chamonix party atmosphere that takes over the whole town for a week or so was not the same. The lack of coloured wrist bands meant you were not sure who was running, or what race, and who were just tourists out running in the days before. There was no 'village' of stalls and promotional stands instead just 2 or 3 including New Balance and Overstim in the field next to the marque. This also affects the prizes and prize giving. At the GRP there was one prize giving for all the next morning. With fewer numbers and sponsors the categories were 10 years not 5, with 3 on the podium not 5, lower value prizes and more local prizes like a side of Bigore dark pork! There also seemed to be a slight touch of chauvanism with the mens prizes being greater than the womens. Still we don't race for the prizes more the glory, or not. One thing that was nicer at the UTMB was a better T shirt and the finishers gillet.
Bumping into friends at the UTMB was far from certain whereas at the GRP it was easy and so very sociable. The buffet after the prize giving was great too- lots of food and lots of variety. Plus in a tiny village almost all beer from just 2 bars about 20m apart.
The main difference for me is the GRP having a 5am start and so only 1 night of running not 2. The small village and low key nature made the GRP a very friendly event. It was slightly harder to get info as they do not publish glossy brochures etc but their web site was good enough. The UTMB is more international and the GRP more French. They had a spread of nationalities but not to the same extent. French was the common language for most, except the Spannish who seemed to prefer English. The other way there was less info was the lack of scrolling screens at major CPs and so supporters were less sure (and for some the SMS system failed) and as a runner I could not see competitors so easily.
The scenery in both is stunning. The UTMB probably goes through higher mountains with glaciers nearby etc but somehow at the GRP I felt I was more in the wilds and not just looking at it in the distance. The terrain at the GRP was a little rougher with a few more technical paths. For many, mine included, it seemed tougher on the feet.
I would cheerfully do either again and would recommend both to any friends wanting a foreign ultra.