[elc]

IMO, I'd say they are both accurate descriptions of why he didn't sign it. I wouldn't make the argument that it was one reason over the other, but after disagreeing with the 3/5ths compromise, the lack of a Bill of Rights was the nail in the coffin for him.

[detsaoT]

Check out Elliot's Debates, the de-facto record of the Constitutional Convention. According to a brief scan of the indices,

Volume 5: MASON, GEORGE. ... Thinks that blacks should, in justice, be counted equally in proportioning representation, but will not insist on it, 302.

By that description alone, it doesn't sound like he's all that upset by it. Instead, he argues a lot over navigational rights, the establishment of courts, the proper jurisdiction of Federal laws, and the like, but he doesn't seem to advocate very strongly for the removal of the three-fifths compromise.