Answer depends on your class. As a pure arbalest xbow (no melee wpf at all, just pure agi/ath/wm) I much prefered 10 WM to some WM and some IF. Run 'n gun someone then find cover to reload that arba as fast as humanly possible.
As a melee build (1h/shield, 2h, pole, pretty much everything) iron flesh is almost NEVER a bad investment. High HP stacks really well with high armor value that most melee prefer anyway. 2H and pole users can do quite well with high WM too. I sometimes like to set my STF character to a high wm and ath and duel with 2d polearms (halberd, english bill, those newer spear-variants). If you can combo quickly enough, two directions is all you need. Also hilarious for instantly guillotining horses and their riders after you rear them.
Of the two skills, I prefer high WM. In general I prefer speed-oriented builds and WM is all about that.