Technically, I'd say Babel should be used on the flanks. He's not a striker. I talked about this with my boy just yesterday. He's not a striker for a very simple reason. He's not good enough. He's not a finisher.
Even more, Babel isn't consistent. Yes, he can have a moment of brilliance and decide the match, but so can more players on that high level.
As for Kuyt, he's no striker either. In form he is, definitely, but no striker. I think he wouldn't do well in a pure upfront role.
Also, his defensive work when employed on the flank exceeds invaluable.