I've got both books and do not regard them to be obscure. Which one helps you more depends on your field of interest. Whereas Thierry's work covers a broader area Cuneo's study focusses on four specific buildings.
Jean-Michel's 'Monuments Armeniens Du Vaspurakan' is in my opinion the essential scholarly work about the entire Vaspurakan region. Its content was first published as a series of articles in the Revue des Études Armeniennes and eventually consolidated in this monograph. Presently several copies are offered on Abebooks starting at approx. USD 53 (http://www.abebooks.com/servlet/SearchResults?an=thierry&sts=t&tn=vaspurakan
). And yes, it's written in French language.
Cuneo's book is in Italian. As my French is far better than my Italian I can't really judge about its scholarly quality. But as I really like his 2 vol. 'Architettura Armena Dal Quarto Al Diciannovesimo Secolo' as a reference work I also appreciate this more specific book on Armenian basilikas.
I come back to my statement above: the main question basically is about your field of interest. And please keep in mind that both works were written a couple of decades ago, so that a lot of information unfortunately do no longer represent the present state of these monuments.
I hope this helps.