We've sailed several times on Viking, in a variety of cabins, and once recently - last cruise before Covid, in 2019 - on Regent in a Penthouse Suite. When comparing Regent to Viking, the closest cabin to a Regent PS is the Viking Penthouse Junior Suite. It's very spacious, and has a bathroom lay-out that is - in our opinion - superior to Regent's bathroom in the PS.
Regent gets very high marks from both loyalists and 'occasional' cruisers for its food quality, and we also found almost all of the food to be very good to excellent. (There were a couple of problems on our particular cruise with less-than-fresh offerings at the pool grill two days in a row, and when we requested the lobster for the following night, we were - surprisingly - told that it couldn't be done. But those are small things overall.) On Viking, we've found the food to be exceptional in all venues at most meals. There is always the occasional small problem, but there are entire cruises where neither of us could honestly find even a small thing to criticize. Important note: This experience comes from the before-times; there are plenty of recent posts about poorer food quality at times on more recent Viking cruises.
Overall, we prefer Viking to Regent for all of the reasons you'll read in other posts...no casino, great passenger-to-space ratios, lovely crew, new and understated Scandinavian ships, and absolutely outstanding resident lecturers. (The lecturers we've experienced across all of our Viking cruises were superior to anything we experienced on our Regent cruise, by a significant margin.) While we did enjoy the all-inclusive aspect of Regent, it's possible to duplicate that to a large extent by adding in Viking's Silver Spirits beverage package.
I hope this helps as you continue your planning!