I'm gonna go DXC but both games top my list of favorite CVs and I love them both for different reasons.
I love the sprite work, graphics, and sounds from Rondo. It's truly a time capsule of where games were at at the time. It was a bit of a turning point for the series and really bridged the gap between the classic and more exploration based titles. I keep a copy of Rondo on my iPhone at all times just in case.
DXC is a great reimaginjng of the game. It's not a carbon copy with prettier graphics and remixed music (althoigh imo the game's effing beautiful!) it's actually like playing a whole new game in some ways. Its a lot darker in tone for one, which is one thing I never liked about Rondo. Everything was so bright, too much, considering the plot. I like the redone versions of some of the stages, particularly Stage 4, thr prison/torture chamber level. That level was always dull to me in Rondo but they gave it a whole new lease on life in DXC. Stage 5' was also incredibly well done. I loved the 2.5d graphics and relished all the little details they put in in some places. The ice and frost in the Dullahan's chamber, the bloody room you fight thr Succubus in in 5', the incredible backgrounds in stage 7, even the little redux of each stage on the stage select screen.
So I love both games, but DXC edges out Rondo. But they're both amazing games!