Potential advisor ask for a paper review as a part of interview process, any difference between journal and conference paper? I am applying for graduate studies in CS, and one of my potential advisors asked me to review one of his papers (listed on his homepage) and send him back a single page report. (discussing strengths and weaknesses)

As long as he did not specify journal or conference paper, is it appropriate to pick one of his conference papers? As it is shorter and regarding the deadline he set for me (1 week), I might be able to analyse it better.

Does it create any negative image of my work and research?
Actually, in CS, conference papers are treated with great respect. Much more so than in some other fields. In CS, most new results are presented in conferences and the proceedings are very important. 

So, contrary to your worry, picking a conference paper would probably be preferred.