Chinese is a tonal language, and it is critical to get the tones right in order to speak properly/use the right words/make yourself understood.
However there are only 4 tones, so it's really not that difficult. I suspect that those who say it's hard are those who aren't prepared to learn to pronounce any language properly. (Something we British are spectacularly 'good' at).
Actually, Mandarin isn't that hard, really. What makes it more complicated than other languages is that you have to learn the characters, so there's an extra step to the language.
I've been learning it for the last 4 years (although very, very part time - have one, 2 hour lesson a week), and thoroughly enjoy it. It is, apparently, very good for developing the conceptual, creative side of your brain.
Having said all of this, the choice between the two languages would depend entirely on why I was needing to learn either of them. To be honest, I'd probably push to do both! They are definitely the languages of the future.