What's better for Ork Nob in Warbiker Mob: Big Choppa or Klaw?
There's a few things to weigh up here.


What else could you get with the points? It's easy to go overboard on your 'hero' units, and forget that you could spend that extra 15 points on more basic troops. With Orks, huge numbers is a core strategy. A few extra Boyz could be much more valuable to you.
Who do you expect to be facing? Ignoring armour is a much more useful ability against Space Marines and Necrons than it is against Dark Eldar or Tyranids. If you regularly play against a few particular armies, then this is particularly relevant. More tactically, consider whether your Ork Nob will be likely to be in a situation where he can make use of armour-ignoring abilities. Will he be attacking tanks, Dreadnoughts, or souped up heroes? Or is the primary role of the squad to act as cannon fodder, or take down more generic troops? Bottom line - there's no point taking expensive custom stuff if you don't make use of it.
What do your other models have? I like giving identical units different weapons, just for variety. This is also handy when you need to pull out a specialised weapon combo for a particular scenario.
What do you like the look of more? This is actually an important point, in my opinion. Warhammer is about aesthetics and cool models just as much as it is about optimising armies and strategies. If you think the Big Choppa looks much better, that's a strong argument for taking that as a weapon!


Without knowing the answers to these questions, I can't give you a straight recommendation. The Klaw is clearly a much more powerful weapon. But whether the cost (both points and aesthetic) is justified depends on all these factors.