If 500 websites get broken you have to fix 500 websites. Thats a lot of work! If 1 website get broken, you have to fix just 1. It is also hard to prove that certain website actually makes $1 per day every day. In 1 day I can register aged 5 years old domain, put some stuff there so that it will looks like a website, buy some cheap traffic for it and sell it to you, claiming that I make $1 every day for 5 years.
So I voter for $500/day website.
But don't buy it for more than its 6-12 months income. It can go down not just to $250 after 3 years, it can be $5 after 1 week and if the main source of traffic is google, it can be even worse. There is always a risk, especially if you don't know the area this website is about.
Conclusion: neither is a good inversment strategy, but 500 $1 websites is definitely worse than 1 $500.