Just my opinion,. but I would take the Riviera deck any day over the Main deck-- go to the Carnival website--pull up the deck plans for the Triumph.
Look at what is over the main deck-- show lounges, pursers desk, dining room, galley ( kitchen, ) etc.
Going up one deck isn't always an " upgrade." - the cabins are the same-By staying on Riviera, you have a passenger deck over you and no lounges, bars, etc. below you.
Plus,you're saving money to spend on something else and that ain't bad !