I know this is an old post that we all dug up but oh well...
Honestly the only real benefit for getting the 450R or YFZ calipers will be cost... you can usually find a pair for $50 and parts are still readily available, that is the advantage. Braking performance as far as I'm aware is the same. Importantly make sure you run a good master cylinder too, I'm personally a fan of the 400ex front master and thats all I run.
Back to the original post... I don't get why everyone is caught up on the running of the 450R setup. Personally I'd only bother with it if I had the stuff laying around. If I were to go buy a setup for any front end right now it would be 400ex spindles, yfz brake calipers, and 400ex master cylinder... Cant beat the fact that its the cheapest and works the best (sometimes you can have your cake and eat it too)