As a fellow F2P’er I currently focus all my gems on Costume Portal myself. But I used to focus on Teltoc until I pulled both Jackal and Falcon in one 10-pull. Still, Costume Portal offers the best bang for the buck for the F2P player. Costumed 4* are bread and butter for me. And as @sleepyhead said, 5* don’t factor into the decision for me. The odds of any 5*, including costume, are astronomically small.
And Happy Cake Day!