Act II mercenaries are capable of only using polearms, spears, and javelins. Polearms are the obvious choice due to the number of good runewords that can be made (particularly Insight and Infinity). The Arreat Summit lists stats for the rangeadder (the weapon-related portion of the melee reach calculation) of each elite polearm (with similar stats for lower-difficulty polearms).
Are Act II mercenaries affected by the weapon reach? Against the same size monster, will my Act II mercenary have a smaller reach with a
Thresher than with a