Honestly. DDC is weird. For me, it seems to mostly revolve around my performances in stages 3 and 4, which I see make up the difficulty bulk of the game. Of course, you can never underestimate the difficulty of stages 5 or 6 either. I worry that people are actually starting to underestimate DDC a bit. Sure, bosses can be learned, but putting it all together in a run is the challenge. I can definitely say that I'm not consistent at getting NMNB Seija fights.
UFO took me much longer to get in the long run. It took a month for me to learn the boss patterns as they're actually meant to be played. (because in the vanilla days, you just bombspam the shit out of the game. Similar to DDC style) However, I had much less experience with Touhou when I was going for the UFO LNB, which was in March.
So I can't really say which is harder. Personally, I would have an easier time with UFO, but I have noticeably more experience with UFO. Most of the time I play UFO, I go for LNBNUFO anyway, but that's a different story.
In DDC, I just have a hard time getting an actual good run going. Unless I'm using SakuyaA or something, I'd find it very hard to get a low miss, no bomb run of the game, like I have of UFO.