I speak (and read and write) both Mandarin and Spanish.
Spanish is a lot easier for an English-speaker to learn.
But Mandarin is, at least IMHO, much more interesting. I enjoy the characters, preferring the traditional ones, coping with the simplified ones.
The most difficult problem I had learning Chinese is that the dominant system of romanization, pinyin, is wholly non-intuitive and conflicting to me as a reader of English. It's frustrating because there are *very* few sounds in Chinese that really couldn't be well-approximated with normal English character order and usage. The exceptions, like the pinyin 'r' sound, could be marked another way (for instance, as the Spanish Ñ.) So learning how to say a word without a native speaker turned out to be a real problem. I got a heck of a boost when a real Chinese restaurant opened in our little town. :)