With regards to load ratings, TireRack has a load rating table which indicates:

81 = 1019 lbs
82 = 1047 lbs
84 = 1102 lbs

You can see that an 84 index tire has a 5.2% increase in load-bearing capacity. A quick search indicates the 1st gen Prius had a curb weight of 2765 pounds. The max GVWR should be in the doorjam, but it's probably a thousand pounds or so. That brings our max vehicle weight with passengers and luggage to 3800 pounds or so. In such a configuration, you've probably achieve near 50/50 weight distribution since the rear seat passengers and bags add a lot more weight to the rear than the front. 3800/4 = 950. That puts you at 91% of max load of the 82 rated tire and 86% of max load of the 84 rated tire.

Deciding on which tire you want to use would be a personal decision based on how comfortable you are running in the upper reaches of a tire's max ratings as well as how often you have a bunch of large people and heavy bags in your car for long highway trips.