Van Thiel's edition has relatively large margins. It has also the advantage that the whole Iliad fits in one volume. Also, some "heretics" still think it's the most reliable scholarly edition at present. (But for a scholarly edition, I would personnally recommend West)
OK, Paul, I take that as a direct and personal slap in the face. Choose your weapon.
West's testimonia (quotations from ancient authors that what how the text read in antiquity) and apparatus (variant readings found in manuscripts, including papyri) are in my view the best in any available edition. (Like van Thiel, he has limited himself to just a few of the hundreds of medieval and later mss., but this is an advantages, not a drawback--for Homer, citing all the mss. would produce a very cluttered apparatus that would be difficult to use, and many of the variants are clearly mistaken and worthless anyway.) His apparatus pays attention to minute details such as accents and breathings.
However, West's text is very idiosyncratic. It reflects his controversial--not necessarily wrong, but not demonstrably right, either--view that the poem was written by a single individual in a Greek community in Asia Minor around 650 BCE. Consequently, his text normalizes spelling to conform to epigraphical evidence from that period and milieu (against the traditional spelling of the medieval mss.). However, West doesn't carry through this practice consistently: he doesn't, for example, render all rough breathings as smooth breathings despite the general belief (which he shares) that the eastern Ionic dialect was already psilotic by the period in which he places the composition of the Iliad. West also brackets some passages on very subjective grounds (including the entire 10th book, which many--probably most--but not all specialists think is somehow not integral to the rest of the Iliad). That is not to say that many of the lines bracketed by West don't deserve to be bracketed: many of them are attested only in a minority of the mss., and do have the odor of interpolations. In fact, some specialists would argue that West doesn't bracket enough of these weakly attested lines.
Van Thiel's edition, in contrast, follows more closely the spelling and other aspects of the text transmitted in the medieval manuscript tradition (and the papyri, as I understand it, though van Thiel's reporting of the papyri is very parsimonious). He doesn't bracket as many lines as West, even those that would be bracketed by most editors.