Much like the boating world, the more power you throw at it, the more incidents you're going to have down the road. At 500 hp you should already be planning on a Mendeola tranny if you want it to last. They're pretty much the #6 drive of the sand world. I would think 500 hp would be a blast without having to worry about the turbos and all the accessories that go along with that system. Not to mention, with that extra 125 hp, it's more wear and tear on the rest of your drivetrain. If I were doing it, I'd go with the 500 hp N/A engine and be done with it. Blown up transmissions and CV joints aren't my idea of fun when you're out in the sand. Pretty much all the guys I go to the dunes with ditched their old turbo rotary monsters in favor of the very engine you're talking about. Now they can dune all weekend and just put gas in it. Before, there was always one or two cars down, even with the big Mendeolas and huge CV's.
Just my $.02