I´ve always tought that the no-crown guard subs are or seem more dressier and elegants that the others. I think that the thickness of the cases makes them more elegants.
But it´s sure that this 5508 or 6538 (probably too much expensive?) are Hits, are true nice exponents of diver watches.
You´re working in a diving thema, and you have probably the best exponents in this thema (jlcs, blancpains ff, longines, rolexes, universal pol, etc..). A no crown guard rolex would work perfectly in your collection. I think that with one nice 5508 you would have a perfect nocrown guard exponent.
In this case... I´d sell the red letter??? umhhh, I think NO. I´ve always thought that the red letters are really nice, because I love this "touch of color" in the dial. But if you think that you are going to sell your r-letter, I can give it a new home
.
Seriously, keep the red letter and give you the pleasure of a nice 5508. Your transitional rolexes are really nice, don´t sell them.
These are my thoughts, hermano.
Let´s see what´s your choice.
Un fuerte abrazo. Nos vemos pronto, Dios Mediante
Emilio