I think klassen could well offer more than Sigurdsson is at the moment.
I think Klaassen will prove to be the better signing, but can understand why he's behind Sig.
Sig's main skill is his delivery, set pieces and when given space to address ball and cross. While he's not used to team mates, he's at least used to speed and physical nature of the league.
Klaassen has great movement and interplay, but you need to build understanding with familiar players, before you can play 1-2s, or help build play. Add to that difficulty of adapting to league, trying to work his way towards a central role in team. Think we won't see best of him any time soon.