The USFS plane is going to have done more low level survey work, is my guess. That might sometimes add more stress to the frame. Then again, the other one has 3000hr more on it and was flown in all kinds of weather. But, mainly at night, so perhaps more in smooth air. Again, it's in the midwest where the storms can get pretty rough in the winters.
Basically, I think it's a wash. Only an inspection will be able to tell if there are any issues. But the hrs in themselves I wouldn't be afraid of per se, but you have to be prepared to be its last owner. Once they get up 10000+hrs, even if there's nothing wrong with them at all, buyers perception will make it very hard to get much value out of them when the day comes to sell it. And that day always comes. But if you're happy with that, it can be a good deal.