Is it better to cite a PhD thesis or an article under review? A collaborator of mine has finished and defended his phd thesis, he has also submitted one chapter of the thesis as an article, and now this article is under review.

I am submitting an article and I want to cite this work, which one of the two should I cite? And if I cite the article should I use "Smith et. al, in review unpublished or pers. communication"

I was thinking that even though not peer-reviewed, a thesis might be better than something under review.
Do you know if his article will be accepted? Would you know so soon enough? If there is a chance that it will be rejected (there always is but you may have a feeling of the quality of his article) I'd cite his thesis.

In any other case I'd cite his peer-reviewed article with Smith et al., under review or Smith et al., submitted to (name of journal). Papers are usually more accessible and perceived to be of higher quality than theses and it would also give him one citation on his article already.

Or, as Pont pointed out, cite both: Statement in your paper (Smith 2016 [this would be the thesis], Smith et al., under review [this would be the pending article]).