Nico Williams is younger than Lookman and he has two big advantages against Lookman: his experience in La Liga is what Lookman does not have and will need time for adaptation to a new league; and his understanding as well as good combination with Lamine Yamal in Spanish national team recent years. It is another disadvantage of Lookkman who will have to get familiarized with Yamal and build up bonds with other new teammates.
I believe transfer prices of Nico Williams and Lookman are not too different and both are expensive. However perhaps Nico Williams will cost Barcelona more money for his wage.
In my opinion, it is best if Barcelona will neither buy Nico Williams nor Lookman in order to focus on improving their club finance and resolve current debt gradually season by season.
The problem is that when Barcelona have Raphinha injured and Lewandowski also injured, Barcelona's attack becomes non-existent, even with Yamal on the field and they will lose a lot, and if the injuries are long-term, then Barcelona will not be able to achieve anything.
In my opinion, Barcelona should make a financial effort to sign Nico Williams and sign some other very skilled player capable of replacing Lewandowski, that way when Lewandowski and Raphinha are injured, the team will continue to be strong in attack and win games.