I always say that I am downgrading when I go from a smaller to larger tank, mostly though that is just because it's my pet-peeve in this hobby everyone but me seems to find bigger is so much better and pretty much always an upgrade - it's like the SUV craze a decade ago, the bigger it was the better, nothing else mattered!
Like Macscale said though it's really what will make you happy! My old biocube was sweet, and like a bonsai forest, I was always pruning and trimming, getting everything to fit just right, and it was a frag producing machine. Was fun and different. It also SPS wise was much faster to make a dense coral scape since the area was smaller.
That being said my new 120 is my dream tank custom made and I don't really have the time for two tanks right now, so I sold my biocube 14... A 120 has, for me, the perfect dimensions of 2:1:1 - anything bigger or smaller is a downgrade to me - all what you like!