He might be playing worse than Iwobi now, but you will not be able to use Iwobi much longer. Young players tend to underperform, but you can improve Anelka much faster (especially if he is an FT), and you will have a well trained player when Iwobi can no longer keep up with your team's quality. It's a good early investment in my honest opinion.

Judging by Iwobi's stats he is developed to play in attack. Anelka has better defense skills (and probably lower passing and crossing than Iwobi), but you should be able to customize him to your personal style of play by training him this season.