Both, I think.
It surely is a nice formalization of Occam's razor, and Solomonoff himself said that he found his distribution while looking for a nice prior over the set of all computable hypothesis. But you can also show that Solomonoff distribution is in a class called dominant semi-measures, which are able to approximate any computable prior with an error that goes to zero very fast.
See for example "Solomonoff induction" by Legg.