What impresses more in a graduate school application for CS: an REU or an honors thesis? I'm a prospective applicant (or will be in the near future) for PhD programs in Computer Science.  I recently did an REU this past summer at a top-ranked CS school and am seriously considering graduate school, albeit after a break so I can be certain which particular area I may want to go into.

While I have my REU as some form of research experience, I'm wondering how much more I will need before I apply to graduate programs.  In my department, there's been a couple who have worked in labs for maybe a year or so (but so far, nothing like two or three years like I have heard at other places), and others in our Honors program are required to do an honors project with a professor at the end.  It's more or less rare that undergrads publish so much where I'm currently doing my undergrad.

While I have done the REU, I haven't gotten much further as far as doing research at my own school goes.  I tried working with a professor one semester during my senior year, but I wasn't able to get much done before I had to stop due to a busy semester next semester.  Generally speaking, is an REU more impressive to graduate admission committees over a senior thesis or doing research at your own institution?
I don't think that there is a definitive answer to the question as asked. It assumes "equal quality", which is unlikely to be easy to measure. Moreover, every reader of an application might favor one over the other, but in a different way from another reader. 

So, I would suggest, treating them of equal "value" on the face. But what will be impressive to a reader is that the applicant has done "something" that required hard work and has high quality. You can achieve this either in developing a thesis or in an REU. 

My advice would be to pick one that interests you and you are likely to work hard at, but also one for which you get some local support. An REU might be slightly favored on the latter idea (support) provided that the research is guided in some way. But even a thesis could be given support if the student meets regularly with a faculty member during development to get some feedback. 

Do something hard. Do it well.