Neither is an absolute (assuming a closed universe), so there are problems with using either. For a long time, the speed of light was considered the only absolute, but IIRC, a pair of Aussie scientists demonstrated last year that there's been some slowing of light (not quite as much as Setterfield calculates, but there) as well.
Gotta love all the monkey wrenches Relativity throws into our attempts to measure the universe, dont'cha?