Although Haaland seems more attractive because he's younger and will give you more years there is also wages you need to consider. He's 20 years old and even if you get a good transfer deal for him this year or wait for the buyout clause next summer, how much would he want?
Say that he wants a deal at £400k per week in his contract. Beside the potential of upsetting other squad members, that wage will only go up when Riola wants to negotiate a new one down the road. You could say he's worth it, but at what point does it become ridiculous money and have to decide to stick or twist to sell him to Barca or Madrid for example?
You can keep him and pay the wages, but how much times you will have to give him an improved contract seeing he's only 20? Also with FFP you could become what Barca or PSG are. A team with 1 or 2 superstars and the rest with average players because you can't afford to bulk up the rest of your squad.
I think Kane coming would mean a decent wage for him, but probably only would only need to give him an improved contract once. Then he'd be off the books at which point you can pay some of you up and coming players.