Both are great watches. While I don't have a submariner, I do own two other Rolexes and the Speedmaster Pro. I have personally tried on the ND SubC on my wrist for a week (switch one of my watches with a friend).
My thoughts are these...The speedy is a classic, a true classic. Any WIS would want to have these three watches, a GS, a Speedy pro and a Rolex Sub.
I baby my GS too much when I wear it. So I tend to be extra careful and very sensitive how I swing my left arm. Bracelet is comfortable and legible dial. Zaratsu polishing makes this a work of art more than a timepiece.
My Speedy...it's nice, it's light, it's timeless and it is so legible. Comfortable bracelet. However at 42mm, a little too big for my 6 1/2 inch wrist.
Rolex SubC ND. It is just solid. just the right weight, slips under the sleeve easily. Very legible, and with the glidelock bracelet, a winner in overall comfort.
So my vote goes to the Rolex hands down. If you drove a car during my teenage years(1990's), you would compare the Rolex to a Volvo. Tough, safe, solid and you will not baby it.
Hope this helps and good luck watch hunting. My advice, try them on first and see how they feel. I know that watch shops will not size the bracelets if you don't buy their watches...look around for friends who has the watches and see if they would allow you to try them...
Patrick