Should I cite a statement as coming from in journal article or from the related PhD thesis?
Personally, I tend to prefer to cite the most recent work, as it is likely to be the most updated and correct.  There is also a good argument to be made for citing the peer-reviewed version (as long as it isn't too much older).  

In general, however, if the two sources are very similar it doesn't matter much which one you cite. There is also no particular need to cite more than one, and I tend to feel it's better to cite only one when the material is basically the same, so that it doesn't look like you're claiming more support than actually exists.