If you have a look at a nutritional comparison (I know rodentpro's site had one), you'll find that older mice are better, so 1 peach may weigh the same as 2 pinks, but will be better for your snakes growth. In the same way, 3 fuzzies will offer less nutritionally for your snake than 1 hopper, even if the combined weight is exactly the same.
So in a long-winded reply, I'd say change to the next size is definately the way to go