Korean is easier to learn to read and write, but much as w1ntermute says upthread it is harder to pronounce. Mandarin is quite regular & systematic in pronunciation; the tone just becomes part of the pronunciation. Grammar is relatively simple compared to Korean. This is an imprecise metric but it also seems to me that Korean has way more syllables for the same words, if nothing else because of all the particles and politeness levels.
But Mandarin reading & writing is punishingly hard. I studied Mandarin for a few years and although I could have some really basic conversations, I never learned to read or write. Korean has felt more rewarding in this regard-- I enjoy sounding out the words and delight in the occasional loanword.
I find Korean easier to speak and be understood. I know some Mandarin, but native speakers don't even know what I'm trying to say to them when I attempt it.
Hmm. I wonder why that is. Did you have a hard time with tones, do you think? Or do you think it's just harder to speak idiomatically? I could definitely see the latter.
My feeling — i.e. this is subjective — is that Mandarin pronunciation is far more regular. In Korean, it's not arbitrary, but there are more rules about pronunciation, and pronunciation is more subtle to my Western ears & tongue.
There are rules for consecutive consonants, where some of them disappear or change entirely (e.g. formal/deferential speech level endings, where p/b turns to m). Some sounds also just get dropped or emphasized based on where they are (e.g. words that end with ㅆ, depending on the next consonant). Then there's plain vs tense vs aspirated, diphthongs, etc. I'm still getting a handle on the various particles but there seem to be around a dozen, not including measure words.
Anyway there's one sort of irregular case in Mandarin, which is 3rd tone followed by 3rd tone. Other than that, if you learn how to say the "ie" as in "xiexie" or "bie", neither "x" nor "ie" or "b" really change depending on what they're paired with.
It may be that in practice, a Korean listener can figure out what sounds you're trying to make more easily than in Mandarin. Anecdotally Mandarin seems to have a lot of cognates because many basic words are one or so syllables + tone. Korean seems to me to have more syllables and more particles, which could help disambiguate. But this is just speculation from someone who is by no means an expert on either language.
But Mandarin reading & writing is punishingly hard. I studied Mandarin for a few years and although I could have some really basic conversations, I never learned to read or write. Korean has felt more rewarding in this regard-- I enjoy sounding out the words and delight in the occasional loanword.