Misnala

Having been on both, I can say that Laura's review is spot on. I did Viking's Rhine cruise (Amsterdam to Basel) in 2015, and Gate 1's Danube cruise (Regensburg to Budapest) this year, and enjoyed both. Overall though, I preferred Gate 1.

Meals:

Breakfast was almost identical. Lunch as well, although wine and beer were not included at lunch on Gate 1. The dinner options on Gate 1 were not as varied as on Viking (one appetizer, rather than a selection between two, etc.) but the meals on both were excellent. Laura mentioned that vegetarians may not have as many options on Gate 1; I'm vegetarian and didn't find that to be the case. All of the meals had at least one vegetarian main course. For the soup and appetizer courses, I was always provided an option.

Excursions:

On both cruises, the ships docked in town in most ports. Both lines offered included, and optional, tours at each stop. On the Viking tour, I participated in most of the offered tours; on Gate 1, I mostly chose to explore on my own. I didn't do any of the optional tours on either cruise, so can't compare. Viking stopped in more locations; Gate 1 stopped in fewer locations, but stayed longer.

Cabin/Crew

On both cruises, I booked a French Balcony room. The cabins were equivalent as far as space and layout. Cabin service was excellent on both lines. The crew was exceptional on both.

On-board activities

About the same. Both had a lounge with piano player, bar, etc. Both had trivia quizzes, and entertainment. Gate 1 had a late-night snack, and tea-time snack, which I don't remember seeing on Viking.

Etc

The majority of the passengers on the Viking cruise were from the US. The Gate 1 cruise had a more varied makeup, with passengers from the US, Britain, Australia, Malaysia, and other countries.

Thanks to everyone posting on this thread! I'm sailing on the Monarch Empress on October 8th but, as Gate 1 has just recently started offering cruises, it's been tough to find many details on the ship, amenities, excursions, etc. I had planned to post a list of questions, but all have been answered in this thread by the very helpful travelers of Cruise Critic. :-)