The best ranked universities tend to award the highest percentage of firsts but, confusing matters, some of the lowest ranked universities aren't far off their percentage.
If the same degree was about as hard at each university, it'd be very unlikely that some of the lowest ranked universities, who generally admit entrants with lower grades on average, would be getting nearly the same percentage.
If the course content looks the same, there's a higher chance that they are closer in difficulty to each other.
There are so many ifs and buts for a potential employer about how hard the degree objectively was. That's why the higher ranked universities are the best bet for employers. They know they don't make their courses deliberately easy. But it all depends on the employer. They're unlikely to pick someone who's lacking personal flair over someone who's not. Down the line, though, people can acquire flair. So go to the best place but that suits you - it must suit your personality as a place to study. If a lower ranked university is really good for a particular subject, that can make a difference though.