NIght in the Ruts was probably the weakest of all of Aerosmith's albums in the 70's (I'd like to see someone point out one that was worse).
Down To Earth was a ~major~ change in the direction Rainbow was going upon the departure of Ronnie J Dio. Neither album is great by any means, but if I had to pick one, I'd probably take Rainbow.
In the grand scheme of things though, since there's a both option, I'm going that route.