Omega fanboy here. Iโve made all the mistakes, so here are my two cents.
I suggest the SMP300 in their favorite color on an OEM stainless mesh strap in the proper length (S, M, or L). I have 2 - the Summer Blue and NTTD. To explain, the SMP300 is bulletproof, suits the active lifestyles of young men, and is a great value. The OEM bracelet is 90โs (ugly), doesnโt taper, and has a massive clasp. Skip the stock bracelet and buy them on rubber, then spec them with the mesh so theyโll have both options. As itโs their first nice watch it should be versatile and show the movement, so opt for neutral colors with an open caseback (black, blue, white, or seaweed). The summer blue and NTTD are awesome and my personal preference, but probably too far out there for their first serious watch.
As a side note, these watches look great on bracelet, rubber, nato, or just about anything. If itโs good enough for Diana to give to Prince William, then it should work for your sons!