Overall I think he's better than Kabak to be honest. Kabak has been better recently but I really don't see what makes him special. He's not particularly strong, fast, or great at positioning. Phillips at least you can easily tell he's only second to VVD in the air. If I have to choose I'd keep Phillips. Might as well save the money for other positions.
That said if his play is deemed not suitable to us, I'd be happy to let him go. A player of his quality deserves regular minutes jn some PL sides rather than sitting on our bench. See Connor Coady for example.
They're very different. Phillips is more of an old school stopper with a purely defensive skillset, whereas Kabak has more to his all round game - less aerially dominant but certainly a better passer and shows more front-foot capabilities with interceptions. Ultimately, they would compliment one another in a different type of set up, but it's never going to work long term with the pair whilst we play the way we do.
It's also worth noting that Kabak has just turned 21 and Phillips is 24. Big age gap in terms of centre halves and their typical development curves. I can see an argument for keeping Phillips round as he's homegrown, but for his own career I think a move to a decent Premier League team would be good. I could easily see him doing well at a Burnley or similar profile club, and we were talking a couple of million for him in the summer, now he'd maybe fetch £10m or so?
He's raised his profile, done really well and exceeded expectations during an extremely testing period. Fair play to the lad. Wish him all the best whether he moves on with our blessing or we decide he's worth keeping around as a fourth/fifth choice homegrown player. Whether Nat will settle for that after starting games at Anfield and in the Champions League, who knows. Would be nice for the lad to feel the warmth the fans have for him with an appearance at an at least partially filled Anfield before the season ends.