What's the difference between 看起来 and 看上去? I see them both used in the same way and both seem to get equal hits in Baidu.
Here are some sample sentences containing each.

http://www.dictall.com/st/60/37/60372038C00.htm

http://www.dictall.com/dictall/result_sentence.jsp?cd=UTF-8&amp;keyword=%E7%9C%8B%E8%B5%B7%E6%9D%A5

Dictionary definitions are similar...

看起来 

①动看来。 看起来得你亲自出马。 ②动从表面上看。 看起来老实，实际上未必。

看上去 

动从外观上估计（多在句子中充当插入成分）。 老人看上去有70岁。
I am not a linguist, and thus can only give you my perspective as a native speaker.

Yes, they are quite similar. So I would only talk about their subtle differences here.

看起来： More often, 看起来 is used when you make a guess by FEELing but not necessarily really physically seeing.

e.g. Today you did not come to school because of a bad fever. 
I may call you and say:"今天怎么没来上学啊？看起来你上病了啊！"
Note here that I did NOT see you at all before I make the guess that you are sick.

看上去： 看上去 is used usually when you express your feeling by really seeing something.

e.g. The moment I see my girlfriend in a beautiful dress. I may go like:"亲爱的，你今天看上去真漂亮！". Note here I can also use 看起来.

To summarize:

You may use 看起来 no matter you really see it or not, but when you see something, I recommend you 看上去. But Please note that it may largely depend on personal preferences.