I ride a 405 and love it. I've not ridden a 506, but in contrast to the above, a mate who has and really likes the 405, was underwhelmed by the longer travel version. I think his take was that the 506 kind of rode like an XC rider's idea of a six-inch bike, a bit like a Whyte 46, which is fine if that's what you want, something that just rolls over stuff in a neutral way, but doesn't have the slightly aggressive slant of the 405 or some of the other, more downhill-orientated six inchers.
I don't know how true that is, but the 405 with Pushed Pikes, air ones, works for everything I ride and takes the sting out of stuff without neutralising it completly, if that makes sense. I'm not sure I'd want any more travel, it feels enough to me.