I only took a year of it in college over a decade ago, so I'm no expert, but I'll give my take on it.
After getting over the initial hurdles of learning the script and the pronunciation, I didn't find the language really too much harder than the French I was also taking at the time. But without a teacher giving you the pronunciation of each word, it would've been harder. Although nowadays there's LingQ so maybe it wouldn't be so bad?
Arabic is similar to Chinese in that there is a large continuum of dialects that share a written language. In either language, as far as I know, when you encounter a new word, you don't know how to pronounce it. In Arabic you have more clues than Chinese because you're given some of the letters, but you really need to hear the word or be familiar with the pronunciation rules of one of the dialects. In that sense I think Arabic is easier than Chinese. Chinese also has tones to deal with, which, at least for me was always a struggle and why I keep starting and stopping the language. Perhaps with more experience in the language, those difficulties ease up and guessing the pronunciation of new words gets easier? I'm not sure.
When you learn Arabic, it seems you have to learn Modern Standard Arabic and one of the local dialects, which can be quite different than MSA, so it kinda feels like you have to learn two languages to get one. In that sense I think Arabic is harder than Chinese, or at least more work.
In the end, they're both challenging in their own way, and as I've never been past A2 in Arabic, or A1 in Chinese (1 semester+off and on later), I can't comment and how the learning curves actually change as you reach higher levels.
It'd be interesting to hear from someone who's native language is far removed from Arabic and Chinese, but has learned both to a reasonably high level.