I have not done a "head-to-head" comparison, but I did compare hunting the same location with both.
Our park has some really deep coins. I've been using the 11" stock coil for years. Got me a 13" ultimate and first trip to the park landed me some keepers. After several months I came to the conclusion that I felt the 13" was slightly deeper but the 11" had the better separation. The difference in depth was about an 1" in favor on the 13" but it was slightly heavier and not as clean in a trashy site.
I ended up selling the ultimate simply because I bought a different detector at the time. I would prefer to have both because they both have places where they shine above the other.