I got ahairline fracture in my 5th metacarpal, splint is enough in this type or I would prefer cast?

Ulnar gutter splint. Splint for 3-4 weeks okay if it immobilizes the joint above and below the fracture site. Your pcp or hand surgeon can ensure that you do not need manipulation and then will customize an ulnar gutter splint for you.
if compliant. If you feel the splint is secure and rigid enough, immobilizing the 5th and sometimes 4th digits as well as extending above the wrist, and there is no angulation, displacement or rotation of the fracture fragment the the splint should be adequate.