There is no way to say which is better. An older building that has been well maintained and has not had moisture issues should be relatively mould free but an older building that has been poorly maintained or has had moisture issues can have massive mould contamination. In turn, newer buildings built well will be relatively mould free and will be less likely to have future mould issues, however not all newer buildings are designed or built well and thus they have the potential to have massive mould contamination problems. The simple rule is look for a building, new or old, that is well built and well maintained. Ensure that the ongoing general maintenance issues are regularly checked and repaired as needed. Adopting a sensible and logical approach will greatly reduce the possibility of having a mould contamination issue in your building.