First off We have sailed on Viking many years ago and were very happy Viking hasn't changed don't re invent the wheel unless it has more spokes. You have been on Viking and were happy cruisers. You are talking about a cruise a year or more in the future, you have plenty of time to do your homework. Since your last Viking cruise was enjoyable start there the go to the other brand and compare. First Viking has many many boats cruising Europe, the Med and around the world, not to diminish this Uniworld brand but lets face it Viking is a premium product in what the do and they do it well with a solid history. Not familiar with Uniworld so can not really comment on the good bad or the ugly in comparing them to Viking.
Homework, Homework, Homework then make an intelligent best guess decision.