As other respondents have already pointed out (one of the advantages of posting late), the texts by Warren Sack and Lev Manovich are problematic in respectively similar and frustrating ways. In both cases, the authors seem to be attempting to stake out their conceptual territory for building a grand, evaluative structure for information visualization as art, but little new construction is in evidence at the end of the articles. Ultimately, I think this is because the questions raised by both Sack and Manovich are reducible to the hoary conundrum of "What purpose does art serve?" Unsurprisingly, they both arrive at slightly different conclusions that appear to prescriptively align with their own inclinations rather than investigate and reveal the motivations of the artists that they cite. Or perhaps, more accurately, neither author adequately explains why their particular conceptions of “good” information visualization are more relevant than any other.
The general direction of Sack's and Manovich's first few pages each superficially describe information visualization as a means of communication through which the artist-designer is attempting to convey information that, either because of excessive breadth or ephemeral nature, is not communicable in an efficient manner using non-visual means. Then, each proceeds to make a different claim—in Sack's case that information visualization should be viewed through the lens of the body politic, and in Manovich's case that information visualization should try to embrace subjectivity. These assertions are then drawn out through the rest of the respective texts without any useful support. Sack cites political works such as Josh On's They Rule as a political information visualization without actually explaining why that is important, necessary, or even effective. Manovich describes Daniel Liebeskind's locative mapping in the Jewish Museum Berlin as arbitrary because he does not know why it was executed in the manner that it was. No evidence is provided that Liebeskind's decision was, in fact arbitrary, but Manovich then proceeds to suggest that information artists embrace arbitrariness because he finds it to be more poetic. But neither Sack nor Manovich really explain why their described approaches are important; it is apparently assumed to be self-evident.
Of course, it is quite possible that I am simply reacting negatively to the prescriptive nature of both texts because information visualization is the field in which I currently (or at least prospectively) reside. While I accept that information visualization art certainly may be examined from a socio-political context (as Sack suggests) and that information visualization may evince poetically arbitrary decisions on the part of the creator (as Manovich maintains), the assertion that one or the other should be inherent to the creative and critical process of the work remains unjustified. Either position could conceivably be argued for virtually any work, which makes the discussion an exercise in frustration.