Although we love Navigator, given the choice for the same itinerary we would choose Mariner.
In our opinion, Mariner has the best layout of public areas of any of the Regent fleet, plus the benefit of an additional speciality restaurant (Chartreuse)
However, be aware that the basic suites on Mariner are snug (significantly smaller than Navigator). For us we prefer a Penthouse or Horizon suite when sailing on Mariner - both grades are an excellent size & layout.
Also, most guests prefer the shower-only suites on Mariner (marked with an asterisk on the deck plans)