Robbie Williams vs Justin Timberlake? They are not even remotely close. And I don't think one is necessarily better than the other.
JT is everything you want in a surface level pop star. Great music, catchy lyrics that require no thought, can't help but put you in a good mood.
Robbie Williams is something else. There's an incredible amount of darkness there for pop music, masked by very simplistic bubblegum music and his own tongue in cheek attitude, but the mask is pretty thin. Justin Timberlake will make great, catchy, deny you like it but always gets you dancing, surface level pop music for years.
Robbie however may very well evolve into something very, very good. I mean that. A lot of his best songs, stripped to their base, cover the exact same themes as an album like Achtung Baby. Temptation/fuck up/redemption - or the opposite - betrayal/fucked up/forgiveness. Robbie's just not 'evolved' as a songwriter or lyricist... yet.
Justin Timberlake comes nowhere near that. Half your average teenage pop themes, half a more adult edgy version of those same themes. And he's fucking brilliant at it.
Musically they are as far apart as two male pop stars could get, lyrics wise it's the same. Their futures will be very different.