If you cherry pick armor used by a Samurai who stuck closely to thier primary function, archery, then yes it will likely have more mobility and less protection than a Knight wearing armor meant to charge headlong into the hail of arrows that Samurai would be firing. However Knights did choose to "dress" for mobility when the situation required it. Again you can cherry pick a particular armor desgin that Knights used for say scaling a wall perhaps, and it will be more mobile and have less protection then one used by a Samurai who supports his failing front line by using his Nodachi to engage Calvery or Spearmen. If you take apex armor From both cultures, maxed out in protection while still offering good mobility, just before firearms changed the world... You will find that they had virtually identical amounts of protection and they could both ride a horse, get knocked off it, roll upon hitting the ground, and Spring back up to thier feet without any overdue hindrance. Because of charging into arrows there are many specialized helms/face plates that severly limit a Knights vision, and because Samurai were archers thier arm and shoulder sections allowed for finer Dexterity. In hand to hand combat the difference would be too small to make a clear decisive difference. Actually it might be safe to say that most full on Knight helms hindered vision more then similar Samurai helms. Likely tied to archery again. I need to research this As for skill yes it's hard to judge and cannot be proven really. But a big misconception many people have is that Knights turned into turtles when grounded. This isn't true. Wresting or grappling was a huge deciding factor in many Knight on Knight engagements. They knew how to handle themselves just fine in this regard.