Both. Simplified characters are officially in use only in Mainland China. For example in Taiwan, Hong Kong, Macau and Singapoore Traditional Characters are still used everywhere.

If you are at the beginning level, learn simplified chinese first. You got more learning resources and the characters taught are standardised. Sometimes, the tranditional chinese characters are diversified in the regions which using them. For example: 为 (simplified) 爲(hongkong traditional) 為(taiwan traditional, also japanese) Also, if you improve your chinese to a certain high level, it wouldn't be very hard to recognise all these characters. Native speaker like us can understand both kind of characters even we usually use only one of them. As you have learned some japanese before, there is a problem that you might mix the two Japanese kanji characters with tranditional chinese characters because they share higher similarity than simplified chinese. Since Chinese and japanese are completely different languages and having different pronounciation on the same character, mixing the characters can be an obstacle. for example: tran：飛行機、中華人民共和國、歸還、 包圍 jp：飛行機、中華人民共和国、帰還、包囲 sim：飞行机、中华人民共和国、归还、包围

除了中国大陆和新加坡使用简体字（simplified chinese characters）之外，其他母语为汉语的国家或地区几乎使用的都是繁体字（traditional chinese characters）。如果你以后在大陆发展机会更多一点建议先学习简体字，等有一定基础且更需要了解更多文化以后再学习繁体字，因为繁体字真的很难写，初学者学起来非常吃力，在大陆生活的年轻人里，用繁体字的少之又少。如果你是在香港澳门或者台湾地区发展的话就得学习繁体字了，因为那些地方通用的就是繁体字。

If you are studying anything other than modern Chinese, stick to traditional. BS has an excellent point. You need to know the cultural differences to gain a deeper understanding of the language, especially in translation! A direct translation example, "My heart & liver".... A westerner without knowledge of Chinese would not know what it means!

Good question - very important.