I'd argue that "better" depends on the situation. I recently rolled a 12/27 character (4 PS, 9 WM, 9 ATH, 1 extra point, 139 2h wpf, 140 polearm wpf), and I was able to switch between my MW Warspear and my MW Katana on the fly for any situation, and let me tell you, it was freaking awesome. They both dealt with most situations really well (with Warspear REALLY only outshining the Katana when Horses were involved, but otherwise pretty equal in usefulness).
The problem with comparing the two is: Bias, situation, particular weapons (Elegent poleaxe is NOT a war spear, but I'd argue that the Greatswords aren't that different from the lower tiered weapons, other than damage/length), personal skill (Some people just can't USE one or the other). In the end, I'd like to argue that it's not which is BETTER, but which it is that You, personally, prefer. I advocate playing ALL of the classes, finding what you like, and then playing that.