I went with the 13" for my Contender barrel. I hunt mostly from a tree stand. Stalking shorter might work out.
When ordering your MGM barrel send in a Dummy Round with the bullet you are planning to use seated to the depth you are planning on seating them. I sent in both a Long Seated 358-180 NOE Cast, and a Long Seated 180 XTP load. My barrel handles both very well.
Another bullet you might look at is the 158 FP/XTP #35780. It might be an even better choice with a shorter barrel.

If you are looking at a 357 Maximum, I am guessing that you know Starline is making brass for it. I am happy Starline took care of that void in brass availability.

Bob