Chinese - Mandarin
I am a native English speaker and writer. I learned Mandarin as a second language and find it quite easy. The tones are hard at first, but after some practice, it's actually pretty easy. Besides, Chinese natives have different accents and it would be hard to tell if someone learned it as a foreign language since there are so many accents out there. Now if you find Mandarin hard, just try to learn Cantonese... There are 6 tones instead of Mandarin's 4 tones. Plus, Mandarin is more widely used.
As for the writing... I find it quite easy to learn after you learned and memorized some characters. Sure, it's a PAIN to write, but after some daily practicing, it'll turn out to be alright. But whoever decided that we have to memorize different versions of some characters should deserve a slap. I don't like having to memorize both simplified and traditional characters; it just adds more difficulty to the writing system. If you don't want aching hands all the time, then I guess simplified as the best for you. Though traditional is also in use in many countries overseas, so it's recommended you learn both. I learned traditional and simplified characters together so I don't have the problem of which one to learn first. I sometimes prefer to write in simplified though it's harder to tell apart the words. I like to read in traditional as it is easier to tell apart the words though writing it is not what I call fun. The only great thing about the writing is that there really isn't much grammar, unlike English. Of course, there is grammar to every language, but Chinese grammar is fairly simple and the particles are easy to understand.
Overall, I think Chinese is a pretty easy language to learn, but most English natives seem to think otherwise. This is just my opinion.