Help with pattern finishing.

I am knitting a “simple” shrug from Vogue Knitting. It is knit in one piece by alternating stockinette and reverse stockinette stitch, 5 rows of each. I want some help with which stitch to use to sew the side and sleeve seams. The pattern is called rolled collar shrug. Thanks in advance.

https://www.ravelry.com/patterns/library/rolled-collar-shrug
Great looking shrug which deserves nice seaming. Mattress stitch works nicely in both stockinette and reverse stockinette, and you would switch between the two as you join.

Thank you so much!